remains on virtually the same point of the retina, and in both cases muscular sensations afford the knowledge that the eye is moving. The shooting-star, however, is perceived to move, and the question remains, why is not the eye in the mirror also seen to move?

F. Ostwald7 refutes the explanation of Graefe from quite different considerations, and gives one of his own, which depends on the geometrical relations subsisting between the axes of vision of the real eye and its reflected image. His explanation is too long to be here considered, an undertaking which indeed the following circumstance renders unnecessary. While it is true that the eye cannot observe the full sweep of its own movement, yet nothing is easier than to observe its movement through the very last part of the arc. If one eye is closed, and the other is brought to within about six inches of an ordinary mirror, and made to describe little movements from some adjacent part of the mirror to its own reflected image, this image can almost without exception be observed as just coming to rest. That is, the very last part of the movement can be seen. The explanation of Ostwald can therefore not be correct, for according to it not alone some parts of the movement, but absolutely all parts alike must remain invisible. It still remains, therefore, to ask why the greater part of the movement eludes observation. The correct explanation will account not only for the impossibility of seeing the first part of the movement but also for the possibility of seeing the remainder.

Apart from the experience of the eye watching itself in a glass, Dodge (loc. citat.) found another fact which strongly suggested anæsthesia. In the course of some experiments on reading, conducted by Erdmann and Dodge, the question came up, how "to explain the meaning of those strangely rhythmic pauses of the eye in reading every page of printed matter." It was demonstrated (ibid., p. 457) "that the rhythmic pauses in reading are the moments of significant stimulation.... If a simple letter or figure is placed between two fixation-points so as to be irrecognizable from both, no eye-movement is found to make it clear, which does not show a full stop between them."

With these facts in view Dodge made an experiment to test the hypothesis of anæsthesia. He proceeded as follows (ibid., p. 458): "A disc of black cardboard thirteen inches in diameter, in which a circle of one-eighth inch round holes, one half inch apart, had been punched close to the periphery all around, was made to revolve at such a velocity that, while the light from the holes fused to a bright circle when the eye was at rest, when the eye moved in the direction of the disc's rotation from one fixation point, seen through the fused circle of light, to another one inch distant, three clear-cut round holes were seen much brighter than the band of light out of which they seemed to emerge. This was only possible when the velocity of the holes was sufficient to keep their images at exactly the same spot on the retina during the movement of the eye. The significant thing is that the individual round spots of light thus seen were much more intense than the fused line of light seen while the eyes were at rest. Neither my assistant nor I was able to detect any difference in brightness between them and the background when altogether unobstructed." Dodge finds that this experiment 'disproves' the hypothesis of anæsthesia.

If by 'anæsthesia' is meant a condition of the retinal end-organs in which they should be momentarily indifferent to excitation by light-waves, the hypothesis is indeed disproved, for obviously the 'three clear-cut round holes' which appeared as bright as the unobstructed background were due to a summation of the light which reached the retina during the movement, through three holes of the disc, and which fell on the same three spots of the retina as long as the disc and the eyeball were moving at the same angular rate. But such a momentary anæsthesia of the retina itself would in any case, from our knowledge of its physiological and chemical structure, be utterly inconceivable.

On the other hand, there seems to be nothing in the experiment which shows that the images of the three holes were present to consciousness just during the movement, rather than immediately thereafter. A central mechanism of inhibition, such as Exner mentions, might condition a central anæsthesia during movement, although the functioning of the retina should remain unaltered. Such a central anæsthesia would just as well account for the phenomena which have been enumerated. The three luminous images could be supposed to remain unmodified for a finite interval as positive after-images, and as such first to appear in consciousness. Inasmuch as 'the arc of eye movements was 4.7°' only, the time would be too brief to make possible any reliable judgment as to whether the three holes were seen during or just after the eye-movement. With this point in view, the writer repeated the experiment of Dodge, and found indeed nothing which gave a hint as to the exact time when the images emerged in consciousness. The results of Dodge were otherwise entirely confirmed.

II. THE PHENOMENON OF 'FALSELY LOCALIZED AFTER-IMAGES.'

A further fact suggestive of anæsthesia during movement comes from an unexpected source. While walking in the street of an evening, if one fixates for a moment some bright light and then quickly turns the eye away, one will observe that a luminous streak seems to dart out from the light and to shoot away in either of two directions, either in the same direction as that in which the eye moved, or in just the opposite. If the eye makes only a slight movement, say of 5°, the streak jumps with the eye; but if the eye sweeps through a rather large arc, say of 40°, the luminous streak darts away in the opposite direction. In the latter case, moreover, a faint streak of light appears later, lying in the direction of the eye-movement.

This phenomenon was probably first described by Mach, in 1886.8 His view is essentially as follows: It is clear that in whatever direction the eye moves, away from its luminous fixation point, the streak described on the retina by the luminous image will lie on the same part of the retina as it would have lain on had the eye remained at rest but the object moved in the opposite direction. Thus, if the eye moves to the right, we should expect the streak to appear to dart to the left. If, however, the streak has not faded by the time the eye has come to rest on a new fixation point (by supposition to the right of the old), we should expect the streak to be localized to the left of this, that is, to the right of the former fixation-point. In order to be projected, a retinal image has to be localized with reference to some point, generally the fixation-point of the eyes; and it is therefore clear that when two such fixation-points are involved, the localization will be ambiguous if for any reason the central apparatus does not clearly determine which shall be the point of reference. With regard to the oppositely moving streak Mach says:9 "The streak is, of course, an after-image,