Chapter II: Preface (2)

It may be said that since the eye does not rotate like the pendulum, from a fulcrum above, the image of _i_ in the case of the moving eye will be distorted as is indicated in Fig. 4, _a_. This is true, but the distortion will be so minute as to be negligible if the pendulum is rather long (say a meter and a half) and the opening _tt_ rather narrow (say not more than ten degrees wide). A merely horizontal movement of the eye will then give a practically exact superposition of the image of _i_ at all moments of the exposure.

Thus much of preliminary discussion to show how, by means of a pendulum, identical stimulations can be given to the moving and to the resting eye. We return to the problem. It is to find out whether a stimulation given during an eye-movement can be perceived if its after-image is so brief as wholly to elapse before the end of the movement. If a period of anæsthesia is to be demonstrated, two observations must be made. First, that the stimulation is bright enough to be _unmistakably visible_ when given to the eye at rest; second, that it is not visible when given to the moving eye. Hence, we shall have three cases.

Case 1. A control, in which the stimulation is proved intense
enough to be seen by the eye at rest.

Case 2. In which the same stimulation is given to the eye
during movement.

Case 3. Another control, to make sure that no change in the
adaptation or fatigue of the eye has intervened during the
experiments to render the eye insensible to the stimulation.

Fig. 5 shows the exact arrangement of the experiment. The figure represents a horizontal section at the eye-level of the pendulum of Fig. 4, with accessories. _E_ is the eye which moves between the two fixation-points _P_ and _P_'. _WONW_ is a wall which conceals the mechanism of the pendulum from the subject. _ON_ is a rectangular hole 9 cm. wide and 7 cm. high, in this wall. _SS_ is the shield which swings with the pendulum, and _BB_ is the background (cf. Fig. 4). When the pendulum is not swinging, a hole in the shield lies behind _ON_ and exactly corresponds with it. Another in the background does the same. The eye can thus see straight through to the light _L_.

Each of these three holes has grooves to take an opaque card, _x_, _y_, or _z_; there are two cards for the three grooves, and they are pierced with holes to correspond to _i_ and _tt_ of Fig. 4. The background _BB_ has a second groove to take a piece of milk-glass _M_. These cards are shown in Fig. 6 (Plate II.) Card _I_ bears a hole 5 cm. high and shaped like a dumb-bell. The diameter of the end-circles (_e_, _e_) is 1.3 cm., and the width of the handle _h_ is 0.2 cm. Card _T_ is pierced by two slits _EE_, _EE_, each 9 cm. long and 1.3 cm. high, which correspond to the two ends of the dumb-bell. These slits are connected by a perforation _H_, 1.5 cm. wide, which corresponds to the handle of the dumb-bell. This opening _EEHEE_ is covered by a piece of ground-glass which serves as a radiating surface for the light.

The distance _EA_ (Fig. 5) is 56 cm., and _PP_' is 40 cm.; so that the arc of eye-movement, that is, the angle _PEP_', is very nearly 40°, of which the 9-cm. opening _ON_ 9° 11'. _SS_ is 2 cm. behind _ON_, and _BB_ 2 cm. behind _SS_; these distances being left to allow the pendulum to swing freely.

It is found under these conditions that the natural speed made by the eye in passing the 9-cm. opening _ON_ is very well approximated by the pendulum if the latter is allowed to fall through 23.5° of its arc, the complete swing being therefore 47°. The middle point of the pendulum is then found to move from _O_ to _N_ in 110[sigma][19]. If the eye sweeps from _O_ to _N_ in the same time, it will be moving at an angular velocity of 1° in 11.98[sigma] (since the 9 cm. are 9° 11' of eye-movement). This rate is much less than that found by Dodge and Cline (_op. cit._, p. 155), who give the time for an eye-movement of 40° as 99.9[sigma], which is an average of only 2.49[sigma] to the degree. Voluntary eye-movements, like other voluntary movements, can of course be slow or fast according to conditions. After the pendulum has been swinging for some time, so that its amplitude of movement has fallen below the initial 47° and therewith its speed past the middle point has been diminished, the eye in its movements back and forth between the fixation-points can still catch the after-image of _i_ perfectly distinct and not at all horizontally elongated, as it would have to be if eye and pendulum had not moved just together. It appears from this that certain motives are able to retard the rate of voluntary movements of the eye, even when the distance traversed is constant.

[19] The speed of the pendulum is measured by attaching a
tuning-fork of known vibration-rate to the pendulum, and
letting it write on smoked paper as the pendulum swings past
the 9-cm. opening.

The experiment is now as follows. The room is darkened. Card _T_ is dropped into groove _z_, while _I_ is put in groove _y_ and swings with the pendulum. One eye alone is used.

Case 1. The eye is fixed in the direction _EA_. The pendulum is allowed to swing through its 47°. The resulting visual image is shown in Fig. 7:1. Its shape is of course like _T_, Fig. 6, but the part _H_ is less bright than the rest because it is exposed a shorter time, owing to the narrowness of the handle of the dumb-bell, which swings by and mediates the exposure. Sheets of milk-glass are now dropped into the back groove of _BB_, until the light is so tempered that part _H_ (Fig. 7:1) is _barely but unmistakably_ visible as luminous. The intensity actually used by the writer, relative to that of _EE_, is fairly shown in the figure. (See Plate III.)

It is clear, if the eye were now to move with the pendulum, that the same amount of light would reach the retina, but that it would be concentrated on a horizontally narrower area. And if the eye moves exactly with the pendulum, the visual image will be no longer like 1 but like 2 (Fig. 7). We do not as yet know how the intensities of _e_, _e_ and _h_ will relatively appear. To ascertain this we must put card _I_ into groove _x_, and let card _T_ swing with the pendulum in groove _y_. If the eye is again fixed in the direction _EA_ (Fig. 5), the retina receives exactly the same stimulation that it would have received before the cards were shifted if it had moved exactly at the rate of the pendulum. In the experiments described, the handle _h_ of this image (Fig. 7:2) curiously enough appears of the same brightness as the two ends _e_, _e_, although, as we know, it is stimulated for a briefer interval. Nor can any difference between _e_, _e_ and _h_ be detected in the time of disappearance of their after-images. These conditions are therefore generous. The danger is that _h_ of the figure, the only part of the stimulation which could possibly quite elapse during the movement, is still too bright to do so.

Case 2. The cards are replaced in their first positions, _T_ in groove _z_, _I_ in groove _y_ which swings. The subject is now asked to make voluntary eye-sweeps from _P_ to _P'_ and back, timing his moment of starting so as to bring his axis of vision on to the near side of opening _ON_ at approximately the same time as the pendulum brings _I_ on the same point. This is a delicate matter and requires practice. Even then it would be impossible, if the subject were not allowed to get the rhythm of the pendulum before passing judgment on the after-images. The pendulum used gives a slight click at each end of its swing, and from the rhythm of this the subject is soon able to time the innervation of his eye so that the exposure coincides with the middle of the eye-movement.

It is true that with every swing the pendulum moves more slowly past _ON_, and the period of exposure is lengthened. This, however, only tends to make the retinal image brighter, so that its disappearance during an anæsthesia would be so much the less likely. The pendulum may therefore be allowed to 'run down' until its swing is too slow for the eye to move with it, that is, too slow for a distinct, non-elongated image of _i_ to be caught in transit on the retina.

With these eye-movements, the possible appearances are of two classes, according to the localization of the after-image. The image is localized either at _A_ (Fig. 5), or at the final fixation-point (_P_ or _P'_, according to the direction of the movement). Localized at _A_, the image may be seen in either of two shapes. First, it may be identical with 1, Fig. 7. It is seen somewhat peripherally, judgment of indirect vision, and is correctly localized at _A_. When the subject's eye is watched, it is found that in this case it moved either too soon or too late, so that when the exposure was made, the eye was resting quietly on one of the fixation-points and so naturally received the same image as in case 1, except that now it lies in indirect vision, the eye being directed not toward _A_ (as in case 1) but towards either _P_ or _P'_.

Second, the image correctly localized may be like 2 (Fig. 7), and then it is seen to move past the opening _ON_. The handle _h_ looks as bright as _e_, _e_. This appearance once obtained generally recurs with each successive swing of the pendulum, and scrutiny of the subject's eye shows it to be moving, not by separate voluntary innervations from _P_ to _P'_ and then from _P'_ to _P_, but continuously back and forth with the swing of the pendulum, much as the eye of a child passively follows a moving candle. This movement is purely reflex,[20] governed probably by cerebellar centers. It seems to consist in a rapid succession of small reflex innervations, and is very different from the type of movement in which one definite innervation carries the eye through its 42°, and which yielded the phenomena with the perimeter. A subject under the spell of this reflex must be exercised in innervating his eye to move from _P_ to _P'_ and back in single, rapid leaps. For this, the pendulum is to be motionless and the eye is not to be stimulated during its movement.

[20] Exner, Sigmund, _Zeitschrift f. Psychologie u. Physiologie
der Sinnesorgane_, 1896, XII., S. 318. 'Entwurf zu einer
physiologischen Erklärung der psychischen Erscheinungen,'
Leipzig u. Wien, 1894, S. 128. Mach, Ernst, 'Beiträge zur
Analyse der Empfindungen,' Jena, 1900, S. 98.

These two cases in which the image is localized midway between _P_ and _P'_ interest us no further. Localized on the final fixation-point, the image is always felt to flash out suddenly _in situ_, just as in the case of the 'correctly localized' after-image streaks in the experiments with the perimeter. The image appears in one of four shapes, Fig. 7: 2 or 3, 4 or 5.

First, the plain or elongated outline of the dumb-bell appears with its handle on the final fixation-point (2 or 3). The image is plain and undistorted if the eye moves at just the rate of the pendulum, elongated if the eye moves more rapidly or more slowly. The point that concerns us is that the image appears _with its handle_. Two precautions must here be observed.

The eye does not perhaps move through its whole 42°, but stops instead just when the exposure is complete, that is, stops on either _O_ or _N_ and considerably short of _P_ or _P'_. It then follows that the exposure is given at the _very last_ part of the movement, so that the after-image of even the handle _h_ has not had time to subside. The experiment is planned so that the after-image of _h_ shall totally elapse during that part of the movement which occurs after the exposure, that is, while the eye is completing its sweep of 42°, from _O_ to _P_, or else from _N_ to _P'_. If the arc is curtailed at point _O_ or _N_, the handle of the dumb-bell will of course appear. The fact can always be ascertained by asking the subject to notice very carefully where the image is localized. If the eye does in fact stop short at _O_ or _N_, the image will be there localized, although the subject may have thoughtlessly said before that it was at _P_ or _P'_, the points he had nominally had in mind.

But the image 2 or 3 may indeed be localized quite over the final fixation-point. In this case the light is to be looked to. It is too bright, as it probably was in the case of Dodge's experiments. It must be further reduced; and with the eye at rest, the control (case I) must be repeated. In the experiments here described it was always found possible so to reduce the light that the distinct, entire image of the dumb-bell (2, Fig. 7) never appeared localized on the final fixation-point, although in the control, _H_, of Fig. 7:1, was always distinctly visible.

With these two precautions taken, the image on the final fixation-point is like either 3, 4, or 5. Shape 5 very rarely appears, while the trained subject sees 4 and 3 each about one half the times; and either may be seen for as many as fifteen times in succession.

Shape 4 is of course exactly the appearance which this experiment takes to be crucial evidence of a moment of central anæsthesia, before the image is perceived and during which the stimulation of the handle _h_ completely elapses. Eight subjects saw this phenomenon distinctly and, after some training in timing their eye-movements, habitually. The first appearance of the handleless image was always a decided surprise to the subject (as also to the writer), and with some eagerness each hastened to verify the phenomenon by new trials.

The two ends (_e_, _e_) of the dumb-bell seem to be of the same intensity as in shape 2 when seen in reflex movement. But there is no vestige whatsoever of a handle. Two of the subjects stated that for them the place where the handle should have been, appeared of a velvety blackness more intense than the rest of the background. The writer was not able to make this observation. It coincides interestingly with that of von Kries,[21] who reports as to the phases of fading after-images, that between the disappearance of the primary image and the appearance of the 'ghost,' a moment of the most intense blackness intervenes. The experiments with the pendulum, however, brought out no ghost.

[21] Von Kries, J., _Zeitschr. f. Psych, u. Physiol. d.
Sinnesorgane_, 1896, XII., S. 88.

We must now enquire why in about half the cases shape 3 is still seen, whereas shape 5 occurs very rarely. Some of the subjects, among whom is the writer, never saw 5 at all. We should expect that with the intensity of _H_ sufficiently reduced 4 and 5 would appear with equal frequency, whereas 3 would be seen no oftener than 2; shape 5 appearing when the eye did not, and 4 when it did, move at just the rate of the pendulum. It is certain that when 4 is seen, the eye has caught just the rate of the pendulum, and that for 3 or 5 it has moved at some other rate. We have seen above (p. 27) that to move with the pendulum the eye must already move decidedly more slowly than Dodge and Cline find the eye generally to move. Nothing so reliable in regard to the rate of voluntary eye-movements as these measurements of Dodge and Cline had been published at the time when the experiments on anæsthesia were carried on, and it is perhaps regrettable that in the 'empirical' approximation of the natural rate of the eye through 40° the pendulum was set to move so slowly.

In any case it is highly probable that whenever the eye did not move at just the rate of the pendulum, it moved _more rapidly_ rather than more slowly. The image is thus horizontally elongated, by an amount which varies from the least possible up to 9 cm. (the width of the opening in _T_), or _even more_. And while the last of the movement (_O_ to _P_, or _N_ to _P'_), in which the stimulation of _H'_ is supposed to subside, is indeed executed, it may yet be done so _rapidly_ that after all _H'_ cannot subside, not even although it is now less intense by being horizontally spread out (that is, less concentrated than the vanished _h_ of shape 4). This explanation is rendered more probable by the very rare appearance of shape 5, which must certainly emerge if ever the eye were to move more slowly than the pendulum.

The critical fact is, however, that shape 4 _does_ appear to a trained subject in about one half the trials--a very satisfactory ratio when one considers the difficulty of timing the beginning of the movement and its rate exactly to the pendulum.

Lastly, in some cases no image appears at all. This was at first a source of perplexity, until it was discovered that the image of the dumb-bell, made specially small so as to be contained within the area of distinct vision, could also be contained on the blind-spot. With the pendulum at rest the eye could be so fixed as to see not even the slight halo which diffuses in the eye and seems to lie about the dumb-bell. It may well occur, then, that in a movement the image happens to fall on the blind-spot and not on the fovea. That this accounts for the cases where no image appears, is proved by the fact that if both eyes are used, some image is always seen. A binocular image under normal convergence can of course not fall on both blind-spots. It may be further said that the shape 4 appears as well when both eyes are used as with only one. The experiment may indeed as well be carried on with both eyes.

Some objections must be answered. It may be said that the image of _h_ happens to fall on the blind-spot, _e_ and _e_ being above and below the same. This is impossible, since the entire image and its halo as well may lie within the blind-spot. If now _h_ is to be on the blind-spot, at least one of the end-circles _e_, _e_ will be there also, whereas shape 4 shows both end-circles of the dumb-bell with perfect distinctness.

Again, it cannot properly be urged that during the movement the attention was distracted so as not to 'notice' the handle. The shape of a dumb-bell was specially chosen for the image so that the weaker part of the stimulation should lie between two points which should be clearly noticed. Indeed, if anything, one might expect this central, connecting link in the image to be apperceptively filled in, even when it did not come to consciousness as immediate sensation. And it remains to ask what it is which should distract the attention.

In this connection the appearance under reflex eye-movement compares interestingly with that under voluntary. If the wall _WONW_ (Fig. 5) is taken from before the pendulum, and the eye allowed to move reflexly with the swinging dumb-bell, the entire image is seen at each exposure, the handle seeming no less bright than the end-circles. Moreover, as the dumb-bell opening swings past the place of exposure and the image fades, although the handle must fade more quickly than the ends, yet this is not discernible, and the entire image disappears without having at any time presented the handleless appearance.

B. Another test for this anæsthesia during movement is offered in the following experiment. It is clear that, just as a light-stimulation is not perceived if the whole retinal process begins and ends during a movement, so also a particular phase of it should not be perceived if that phase can be given complete within the time of the movement. The same pendulum which was used in the previous experiment makes such a thing possible. If in place of the perforated dumb-bell the pendulum exposes two pieces of glass of nearly complementary colors, one after the other coming opposite the place of exposure, the sensations will fuse or will not fuse according as the pendulum swings rapidly or slowly. But now a mean rate of succession can be found such as to let the first color be seen pure before the second is exposed, and then to show the second fused with the after-image of the first. Under some conditions the second will persist after the first has faded, and will then itself be seen pure. Thus there may be three phases in consciousness. If the first color exposed is green and the second red, the phases of sensation will be green, white, and perhaps red. These phases are felt to be not simultaneous but successive. A modification of this method is used in the following experiment. (See Fig. 8, Plate IV.)

_T_ and _I_ here correspond to the cards _T_ and _I_ of Fig. 6. _T_ consists of a rectangular opening, 9×5 cm., which contains three pieces of glass, two pieces of green at the ends, each 2.8 cm. wide and 7 cm. high, and a piece of red glass in the middle 3.4 cm. wide and only 1.5 cm. high, the space above and below this width being filled with opaque material. The shape of the image is determined as before by the hole in _I_, which now, instead of being a dumb-bell, is merely a rectangular hole 2 cm. wide and 5 cm. high. Exactly as before, _T_ is fixed in the background and _I_ swings with the pendulum, the eye moving with it.

The speed of the pendulum must be determined, such that if _I_ lies in the front groove (Fig. 5, _x_) and the eye is at rest, the image will clearly show two phases of color when _T_ swings past on the pendulum. With _T_ and _I_ as described above, a very slow pendulum shows the image green, red (narrow), and green, in succession. A very fast pendulum shows only a horizontal straw-yellow band on a green field (Fig. 8:5). There is but one phase and no feeling of succession. Between these two rates is one which shows two phases--the first a green field with a horizontal, reddish-orange band (Fig. 8:3), the second quickly following, in which the band is straw-yellow (5). It might be expected that this first phase would be preceded by an entirely green phase, since green is at first exposed. Such is however not the case. The straw-yellow of the last phase is of course the fusion-color of the red and green glasses. It would be gray but that the two colors are not perfectly complementary. Since the arrangement of colors in _T_ is bilaterally symmetrical, the successive phases are the same in whichever direction the pendulum swings.

It is desirable to employ the maximum rate of pendulum which will give the two phases. For this the illumination should be very moderate, since the brighter it is, the slower must be the pendulum. With the degree of illumination used in the experiments described, it was found that the pendulum must fall from a height of only 9.5° of its arc: a total swing of 19°. The opening of _T_, which is 9 cm. wide, then swings past the middle point of _I_ in 275[sigma].

Now when the eye moves it must move at this rate. If the eye is 56 cm. distant from the opening, as in the previous case, the 9 cm. of exposure are 9° 11' of eye-movement, and we saw above that 9° 11' in 110[sigma] is a very slow rate of movement, according to the best measurements. Now it is impossible for the eye to move so slowly as 9° 11' in 275[sigma]. If, however, the eye is brought nearer to the opening, it is clear that the 9 cm. of exposure become more than 9° 11' of eye-movement. Therefore the eye and the fixation-points are so placed that _EA_ (Fig. 5) = 26 cm. and _PP'_ = 18 cm. The total eye-movement is thus 38° 11', of which the nine-centimeter distance of exposure is 19° 38'. Now the eye is found to move very well through 19° 38' in 275[sigma], although, again, this is much more than a proportionate part of the total time (99.9[sigma]) given by Dodge and Cline for a movement of the eye through 40°. The eye is in this case also moving slowly. As before, it is permissible to let the pendulum run down till it swings too slowly for the eye to move with it; since any lessened speed of the pendulum only makes the reddish-orange phase more prominent.

As in the experiment with the dumb-bell, we have also here three cases: the control, the case of the eye moving, and again a control.

Case 1. _T_ swings with the pendulum. _I_ is placed in the front groove, and the eye looks straight forward without moving. The pendulum falls from 9.5° at one side, and the illumination is so adjusted that the phase in which the band is reddish-orange, is _unmistakably_ perceived before that in which it is straw-yellow. The appearance must be 3 followed by 5 (Fig. 8).

Case 2. _T_ is fixed in the background, _I_ on the pendulum, and the phenomena are observed with the eye moving.

Case 3. A repetition of case 1, to make sure that no different adaptation or fatigue condition of the eye has come in to modify the appearance of the two successive phases as at first seen.

The possible appearances to the moving eye are closely analogous to those in the dumb-bell experiment. If the eye moves too soon or too late, so that it is at rest during the exposure, the image is like _T_ itself (Fig. 8) but somewhat fainter and localized midway between the points _P_ and _P'_. If the eye moves reflexly at the rate of the pendulum, the image is of the shape _i_ and shows the two phases (3 followed by 5). It is localized in the middle and appears to move across the nine-centimeter opening.

A difficulty is met here which was not found in the case of the dumb-bell. The eye is very liable to come to a full stop on one of the colored surfaces, and then to move quickly on again to the final fixation-point. And this happens contrary to the intention of the subject, and indeed usually without his knowledge. This stopping is undoubtedly a reflex process, in which the cerebellar mechanism which tends to hold the fixation on any bright object, asserts itself over the voluntary movement and arrests the eye on the not moving red or green surface as the exposure takes place. A comparable phenomenon was found sometimes in the experiment with the dumb-bell, where an eye-movement commenced as voluntary would end as a reflex following of the pendulum. In the present experiment, until the subject is well trained, the stopping of the eye must be watched by a second person who looks directly at the eye-ball of the subject during each movement. The appearances are very varied when the eye stops, but the typical one is shown in Fig. 8:1. The red strip _AB_ is seldom longer and often shorter than in the figure. That part of it which is superposed on the green seldom shows the orange phase, being almost always of a pure straw-yellow. The localization of these images is variable. All observations made during movements in which the eye stops, are of course to be excluded.

If now the eye does not stop midway, and the image is not localized in the center, the appearance is like either 2, 4, or 5, and is localized over the final fixation-point. 2 is in all probability the case of the eye moving very much faster than the pendulum, so that if the movement is from left to right, the right-hand side of the image is the part first exposed (by the uncovering of the left-hand side of _T_), which is carried ahead by the too swift eye-movement and projected in perception on the right of the later portion. 3 is the case of the eye moving at very nearly but not quite the rate of the pendulum. The image which should appear 2 cm. wide (like the opening _i_) appears about 3 cm. wide. The middle band is regularly straw-yellow, extremely seldom reddish, and if we could be sure that the eye moves more slowly than the pendulum, so that the succession of the stimuli is even slower than in the control, and the red phase is surely given, this appearance (3) would be good evidence of anæsthesia during which the reddish-orange phase elapses. It is more likely, however, that the eye is moving faster than the pendulum, but whether or not so inconsiderably faster as still to let the disappearance of the reddish phase be significant of anæsthesia, is not certain until one shall have made some possible but tedious measurements of the apparent width of the after-image. Both here and in the following case the _feeling of succession_, noticeable between the two phases when the eye is at rest, has _disappeared with the sensation of redness_.

The cases in which 5 is seen are, however, indisputably significant. The image is apparently of just the height and width of _i_, and there is not the slightest trace of the reddish-orange phase. The image flashes out over the final fixation-point, green and straw-yellow, just as the end-circles of the dumb-bell appeared without their handle. The rate of succession of the stimuli, green--red--green, on the retina, is identical with that rate which showed the two phases to the resting eye: for the pendulum is here moving at the very same rate, and the eye is moving exactly with the pendulum, as is shown by the absence of any horizontal elongation of the image seen. The trained subject seldom sees any other images than 4 and 5, and these with about equal frequency, although either is often seen in ten or fifteen consecutive trials. As in the cases of the falsely localized images and of the handleless dumb-bell, movements of both eyes, as well as of the head but not the eyes, yield the same phenomena. It is interesting again to compare the appearance under reflex movement. If at any time during the experiments the eye is allowed to follow the pendulum reflexly, the image is at once and invariably seen to pass through its two phases as it swings past the nine-centimeter opening.

The frequent and unmistakable appearance of this band of straw-yellow on a non-elongated green field _without the previous phase in which the band is reddish-orange_, although this latter was unmistakable when the same stimulation was given to the eye at rest, is authenticated by eight subjects. _This appearance, together with that of the handleless dumb-bell, is submitted as a demonstration that during voluntary movements of the eyes, and probably of the head as well, there is a moment in which stimulations are not transmitted from the retina to the cerebral cortex, that is, a moment of central anæsthesia_. The reason for saying 'and _probably_ of the head as well,' is that although the phenomena described are gotten equally well from movements of the head, yet it is not perfectly certain that when the head moves the eyes do not also move slightly within the head, even when the attempt is made to keep them fixed.

Most of the criticisms which apply to this last experiment apply to that with the dumb-bell and have already been answered. There is one however which, while applying to that other, more particularly applies here. It would be, that these after-images are too brief and indistinct to be carefully observed, so that judgments as to their shape, size, and color are not valid evidence. This is a perfectly sensible criticism, and a person thoroughly convinced of its force should repeat the experiments and decide for himself what reliance he will place on the judgments he is able to make. The writer and those of the subjects who are most trained in optical experiments find the judgments so simple and easily made as not to be open to doubt.

In the first place, it should be remembered that only those cases are counted in which the movement was so timed that the image was seen in direct vision, that is, was given on or very near the fovea. In such cases a nice discrimination of the shape and color of the images is easily possible.

Secondly, the judgments are in no case quantitative, that is, they in no case depend on an estimate of the absolute size of any part of the image. At most the proportions are estimated. In the case of the dumb-bell the question is, Has the figure a handle? The other question, Are the end-circles horizontally elongated? has not to be answered with mathematical accuracy. It is enough if the end-circles are approximately round, or indeed are narrower than 9 cm. horizontally, for at even that low degree of concentration the handle was still visible to the resting eye. Again, in the experiment with the color-phases, only two questions are essential to identify the appearance 5: Does the horizontal yellow band extend quite to both edges of the image? and, Is there certainly no trace of red or orange to be seen? The first question does not require a quantitative judgment, but merely one as to whether there is any green visible to the right or left of the yellow strip. Both are therefore strictly questions of quality. And the two are sufficient to identify appearance 5, for if no red or orange is visible, images 1, 2, and 3 are excluded; and if no green lies to the right or left of the yellow band, image 4 is excluded. Thus if one is to make the somewhat superficial distinction between qualitative and quantitative judgments, the judgments here required are qualitative. Moreover, the subjects make these judgments unhesitatingly.

Finally, the method of making judgments on after-images is not new in psychology. Lamansky's well-known determination of the rate of eye-movements[22] depends on the possibility of counting accurately the number of dots in a row of after-images. A very much bolder assumption is made by Guillery[23] in another measurement of the rate of eye-movements. A trapezoidal image was generated on the moving retina, and the after-image of this was projected on to a plane bearing a scale of lines inclining at various angles. On this the degree of inclination of one side of the after-image was read off, and thence the speed of the eye-movement was calculated. In spite of the boldness of this method, a careful reading of Guillery's first article cited above will leave no doubt as to its reliability, and the accuracy of discrimination possible on these after-images.

[22] Lamansky, S., (Pflüger's) Archiv f. d. gesammte
Physiologie, 1869, II., S. 418.

[23] Guillery, (Pflüger's) Archiv f. d. ges. Physiologie, 1898,
LXXI., S. 607; and 1898, LXXIII., S. 87.

As to judgments on the color and color-phases of after-images, there is ample precedent in the researches of von Helmholtz, Hering, Hess, von Kries, Hamaker, and Munk. It is therefore justifiable to assume the possibility of making accurately the four simple judgments of shape and color described above, which are essential to the two proofs of anæsthesia.

V. SUMMARY AND COROLLARIES OF THE EXPERIMENTS, AND A PARTIAL, PHYSIOLOGICAL INTERPRETATION OF THE CENTRAL ANÆSTHESIA.

We have now to sum up the facts given by the experiments. The fact of central anæsthesia during voluntary movement is supported by two experimental proofs, aside from a number of random observations which seem to require this anæsthesia for their explanation. The first proof is that if an image of the shape of a dumb-bell is given to the retina during an eye-movement, and in such a way that the handle of the image, while positively above the threshold of perception, is yet of brief enough duration to fade completely before the end of the movement, it then happens that both ends of the dumb-bell are seen but the handle not at all. The fact of its having been properly given to the retina is made certain by the presence of the now disconnected ends.

The second proof is that, similarly, if during an eye-movement two stimulations of different colors are given to the retina, superposed and at such intensity and rate of succession as would show to the resting eye two successive phases of color (in the case taken, reddish-orange and straw-yellow), it then happens that the first phase, which runs its course and is supplanted by the second before the movement is over, is not perceived at all. The first phase was certainly given, because the conditions of the experiment require the orange to be given if the straw-yellow is, since the straw-yellow which is seen can be produced only by the addition of green to the orange which is not seen.

These two phenomena seem inevitably to demonstrate a moment during which a process on the retina, of sufficient duration and intensity ordinarily to determine a corresponding conscious state, is nevertheless prevented from doing so. One inclines to imagine a retraction of dendrites, which breaks the connection between the central end of the optic nerve and the occipital centers of vision.

The fact of anæsthesia demonstrated, other phenomena are now available with further information. From the phenomena of the 'falsely localized' images it follows that at least in voluntary eye-movements of considerable arc (30° or more), the anæsthesia commences appreciably later than the movement. The falsely localized streak is not generated before the eye moves, but is yet seen before the correctly localized streak, as is shown by the relative intensities of the two. The anæsthesia must intervene between the two appearances. The conjecture of Schwarz, that the fainter streak is but a second appearance of the stronger, is undoubtedly right.

We know too that the anæsthesia depends on a mechanism central of the retina, for stimulations are received during movement but not transmitted to consciousness till afterward. This would be further shown if it should be found that movements of the head, no less than those of the eyes, condition the anæsthesia. As before said, it is not certain that the eyes do not move slightly in the head while the head moves. The movement of the eyes must then be very slight, and the anæsthesia correspondingly either brief or discontinuous. Whereas, the phenomena are the same when the head moves 90° as when the eyes move that amount. It seems probable, then, that voluntary movements of the head do equally condition the anæsthesia.

We have seen, too, that in reflex eye-or head-movements no anæsthesia is so far to be demonstrated. The closeness with which the eye follows the unexpected gyrations of a slowly waving rush-light, proves that the reflex movement is produced by a succession of brief impulses (probably from the cerebellum), each one of which carries the eye through only a very short distance. It is an interesting question, whether there is an instant of anæsthesia for each one of these involuntary innervations--an instant too brief to be revealed by the experimental conditions employed above. The seeming continuity of the sensation during reflex movement would of course not argue against such successive instants of anæsthesia, since no discontinuity of vision during voluntary movement is noticeable, although a relatively long moment of anæsthesia actually intervenes.

But decidedly the most interesting detail about the anæsthesia is that shown by the extreme liability of the eye to stop reflexly on the red or the green light, in the second experiment with the pendulum. Suppose the eye to be moving from _P_ to _P'_ (Fig. 5); the anæsthesia, although beginning later than the movement, is present when the eye reaches _O_, while it is between _O_ and _N_, that is, during the anæsthetic moment, that the eye is reflexly caught and held by the light. This proves again that the anæsthesia is not retinal, but it proves very much more; namely, that _the retinal stimulation is transmitted to those lower centers which mediate reflex movements, at the very instant during which it is cut off from the higher, conscious centers_. The great frequency with which the eye would stop midway in its movements, both in the second pendulum-experiment and in the repetition of Dodge's perimeter-test, was very annoying at the time, and the observation cannot be questioned. The fact of the habitual reflex regulation of voluntary movements is otherwise undisputed. Exner[24] mentions a variety of similar instances. Also, with the moving dumb-bell, as has been mentioned, the eye having begun a voluntary sweep would often be caught by the moving image and carried on thereafter reflexly with the pendulum. These observations hang together, and prove a connection between the retina and the reflex centers even while that between the retina and the conscious centers is cut off.

[24] Exner, Sigmund, 'Entwurf zu einer physiologischen
Erklärung der psychischen Erscheinungen,' Leipzig und Wien,
1894, S. 124-129.

But shall we suppose that the 'connection' between the retina and the conscious centers is cut off during the central anæsthesia? All that the facts prove is that the centers are at that time not conscious. It would be at present an unwarrantable assumption to make, that these centers are therefore disconnected from the retina, at the optic thalami, the superior quadrigeminal bodies, or wheresoever. On broad psychological grounds the action-theory of Münsterberg[25] has proposed the hypothesis that cerebral centers fail to mediate consciousness not merely when no stimulations are transmitted to them, but rather when the stimulations transmitted are not able to pass through and out. The stimulation arouses consciousness when it finds a ready discharge. And indeed, in this particular case, while we have no other grounds for supposing stimulations _to_ the visual centers to be cut off, we do have other grounds for supposing that egress _from_ these cells would be impeded.

[25] Münsterberg, Hugo, 'Grundzüge der Psychologie,' Leipzig,
1900, S. 525-561.

The occipital centers which mediate sensations of color are of course most closely associated with those other centers (probably the parietal) which receive sensations from the eye-muscles and which, therefore, mediate sensations which furnish space and position to the sensations of mere color. Now it is these occipital centers, mediators of light-sensations merely, which the experiments have shown most specially to be anæsthetic. The discharge of such centers means particularly the passage of excitations on to the parietal localization-centers. There are doubtless other outlets, but these are the chief group. The movements, for instance, which activity of these cells produces, are first of all eye-movements, which have to be _directly_ produced (according to our present psychophysical conceptions) by discharges from the centers of eye-muscle sensation. The principal direction of discharge, then, from the color-centers is toward the localization-centers.

Now the experiment with falsely and correctly localized after-images proves that before the anæsthesia all localization is with reference to the point of departure, while afterwards it is with reference to the final fixation-point. The transition is abrupt. During the anæsthesia, then, the mechanism of localization is suffering a readjustment. It is proved that during this interval of readjustment in the centers of eye-muscle sensation the way is closed to oncoming discharges from the color-centers; but it is certain that any such discharge, during this complicated process of readjustment, would take the localization-centres by surprise, as it were, and might conceivably result in untoward eye-movements highly prejudicial to the safety of the individual as a whole. The much more probable event is the following:

Although Schwarz suggests that the moment between seeing the false and seeing the correct after-image is the moment that consciousness is taken up with 'innervation-feelings' of the eye-movement, this is impossible, since the innervation-feelings (using the word in the only permissible sense of remembered muscle-sensations) must _precede_ the movement, whereas even the first-seen, falsely localized streak is not generated till the movement commences. But we do have to suppose that during the visual anæsthesia, muscle-sensations of _present_ movement are streaming to consciousness, to form the basis of the new post-motum localization. And these would have to go to those very centers mentioned above, the localization-centers or eye-muscle sensation centers. One may well suppose that these incoming currents so raise the tension of these centers that for the moment no discharge can take place thither from other parts of the brain, among which are the centers for color-sensations. The word 'tension' is of course a figure, but it expresses the familiar idea that centers which are in process of receiving peripheral stimulations, radiate that energy _to_ other parts of the brain (according to the neural dispositions), and probably do not for the time being receive communications therefrom, since those other parts are now less strongly excited. It is, therefore, most probable that during the incoming of the eye-muscle sensations the centers for color are in fact not able to discharge through their usual channels toward the localization-centers, since the tension in that direction is too high. If, now, their other channels of discharge are too few or too little used to come into question, the action-theory would find in this a simple explanation of the visual anæsthesia.

The fact that the anæsthesia commences appreciably later than the movement so far favors this interpretation. For if the anæsthesia is conditioned by high tension in the localization-centers, due to incoming sensations from the eye-muscles, it could not possibly commence synchronously with the movement. For, first the sensory end-organs in the eye-muscles (or perhaps in the ligaments, surfaces of the eye-sockets, etc.) have their latent period; then the stimulation has to travel to the brain; and lastly it probably has to initiate there a summation-process equivalent to another latent period. These three processes would account very readily for what we may call the latent period of the anæsthesia, as observed in the experiments. It is true that this latent period was observed only in long eye-and head-movements, but the experiments were not delicate enough in this particular to bring out the finer points.

Finally, the conditioning of anæsthesia by movements of the head, if really proved, would rather corroborate this interpretation. For of course the position of the head on the shoulders is as important for localization of the retinal picture as the position of the eyes in the head, so that sensations of head-movements must be equally represented in the localization centers; and head movements would equally raise the tension on those centers against discharge-currents from the color-centers.

The conclusion from the foregoing experiments is that voluntary movements of the eyes condition a momentary, visual, central anæsthesia.

* * * * *

TACTUAL ILLUSIONS.

BY CHARLES H. RIEBER.

I.

Many profound researches have been published upon the subject of optical illusions, but in the field of tactual illusions no equally extensive and serious work has been accomplished. The reason for this apparent neglect of the illusions of touch is obviously the fact that the studies in the optical illusions are generally thought to yield more important results for psychology than corresponding studies in the field of touch. Then, too, the optical studies are more attractive by reason of the comparative ease and certainty with which the statistics are gathered there. An optical illusion is discovered in a single instance of the phenomenon. We are aware of the illusion almost immediately. But in the case of most of the illusions of touch, a large number of experiments is often necessary in order to reveal any approximately constant error in the judgments. Nevertheless, it seems to me that the factors that influence our judgments of visual space, though their effects are nearly always immediately apparent, are of no more vital significance for the final explanation of the origin of our notion of space than the disturbing factors in our estimations of tactual space whose effects are not so open to direct observation.

The present investigation has for its main object a critical examination of the tactual illusions that correspond to some of the well-known optical illusions, in the hope of segregating some of the various disturbing factors that enter into our very complex judgments of tactual space. The investigation has unavoidably extended into a number of near-lying problems in the psychology of touch, but the final object of my paper will be to offer a more decisive answer than has hitherto been given to the question, _Are the optical illusions also tactual illusions, or are they reversed for touch?_

Those who have given their attention to illusions of sight and touch are rather unequally divided in their views as to whether the geometrical optical illusions undergo a reversal in the field of touch, the majority inclining to the belief that they are reversed. And yet there are not wanting warm adherents of the opposite view. A comparison of the two classes of illusions, with this question in view, appears therefore in the present state of divergent opinion to be a needed contribution to experimental psychology. Such an experimental study, if it succeeds in finding the solution to this debate, ought to throw some further light upon the question of the origin of our idea of space, as well as upon the subject of illusions of sense in general. For, on the one hand, if touch and sight function alike in our judgment of space, we should expect that like peripheral disturbances in the two senses would cause like central errors in judgment, and every tactual analogue of an optical illusion should be found to correspond both in the direction of the error and, to a certain extent, quantitatively with the optical illusion. But if, on the other hand, they are in their origin and in their developed state really disparate senses, each guided by a different psychological principle, the illusion in the one sense might well be the reverse of the corresponding illusion in the other sense. Therefore, if the results of an empirical study should furnish evidence that the illusions are reversed in passing from one field to the other, we should be obliged to conclude that we are here in the presence of what psychologists have been content to call the 'unanalyzable fact' that the two senses function differently under the same objective conditions. But if, on the contrary, it should turn out that the illusions are not reversed for the two senses, then the theory of the ultimate uniformity of the psychical laws will have received an important defence.

These experiments were carried on in the Harvard Psychological Laboratory during the greater part of the years 1898-1901. In all, fifteen subjects coöperated in the work at different times.

The experimental work in the direction of a comparison of the optical illusions with the tactual illusions, to the time of the present investigation, has been carried on chiefly with the familiar optical illusion of the overestimation of filled space. If the distance between two points be divided into two equal parts by a point midway between them, and the one of the halves be filled with intermediate points, the filled half will, to the eye, appear longer than the open half. James[1] says that one may easily prove that with the skin we underestimate a filled space, 'by taking a visiting card, and cutting one edge of it into a saw-toothed pattern, and from the opposite edge cutting out all but two corners, and then comparing the feelings aroused by the two edges when held against the skin.' He then remarks, 'the skin seems to obey a different law here from the eye.' This experiment has often been repeated and verified. The most extensive work on the problem, however, is that by Parrish.[2] It is doubtless principally on the results of Parrish's experiments that several authors of text-books in psychology have based their assertions that a filled space is underestimated by the skin. The opposite conclusion, namely, that the illusion is not reversed for the skin, has been maintained by Thiéry,[3] and Dresslar.[4] Thiéry does not, so far as I know, state the statistics on which he bases his view. Dresslar's experiments, as Parrish has correctly observed, do not deal with the proper analogue of the optical illusion for filled space. The work of Dresslar will be criticised in detail when we come to the illusions for active touch.

[1] James, William: 'Principles of Psychology,' New York, 1893,
Vol. II., p. 141.

[2] Parrish, C.S.: _Amer. Journ. of Psy._, 1895, Vol. VI., p.
514.

[3] Thiéry, A.: _Philos. Studien_, 1896, Bd. XII., S. 121.

[4] Dresslar, F.B.: _Amer. Journ. of Psy._, 1894, Vol. VI., p.
332.