The arc ran on an alternating circuit of 60 phases per second, and owing to these interruptions of the illumination the curve of the corneal image showed on the negative as a dotted line in which the distance between any two dots represented one sixtieth of a second. Since the constancy of this alternation in the current has been measured in the Jefferson Physical Laboratory (of Harvard), and found to vary within a few tenths of one per cent only, the spacing of the dots on the negatives formed the most convenient possible means for determining the durations of the nystagmiform movements. These dots are shown in Figs. 3, 4, and 5 (Plates I and II).
PLATE I.
(By an error Fig. 4 is shown reversed; the lettering is correct.)
Fig. 3 shows a portion of one of the films. The two curves are to be read from below upwards; but at the bottom is a photograph of the slit (showing a part of the subject's face) taken when the drum had made a little over one revolution and had come back to rest. Hence below the image of the slit, the curve of corneal reflection is doubled. "Right" and "Left" refer to the subject's right and left sides, so that the reader looks into the subject's face from in front. In the picture of the slit, the place on the cornea of the corneal reflection is shown; and also a minor reflection, which as may be seen traced no curve, from some other source of light. The fine line that crosses the slit horizontally is the image of the thread, above mentioned, which was used in adjusting the head. The time-dots are seen to be perfectly distinct, so that they could be accurately read with the help of a jeweller's eyeglass. Fig. 4 shows another part of the same negative, a portion subsequent to the single eye-curves of Fig. 3, that is, a continuation vertically upwards of Fig. 3. The rotation had been from the subject's left to his right, a direction that will be termed "clockwise" throughout this paper, and it can be seen that the quick eye-movements are toward the subject's left, while the slow are towards his right: had the photograph been taken during the rotation, the directions of the quick and slow movements would have been reversed. Two points may be observed in this figure which the tables will also bring out,—that the two eyes move together, and that as the nystagmus subsides the quick eye-movements become less frequent but endure no longer, or in other words, the slow movements alone increase in duration. The corneal reflection does not accurately show the amplitude of the movements; but direct inspection of a subject's eyes, as the nystagmus dies away, shows that generally (but perhaps not always) the amplitudes of both quick and slow movements decrease together. When this is the case, it follows that at the end of the nystagmus the rate of the slow movements decreases very much faster than that of the rapid movements.
Readable negatives were obtained from four, out of six subjects. Of such negatives there are fourteen, ten of which are of eye-movements after rotation clockwise, and four after rotation anti-clockwise. This distribution is accidental, for the rotations in each direction were about equal in number. With the exceptions to be noted later all the negatives exhibit the same features, so that of the fourteen only four examples are given in full in the tables; while for the others merely the averages of the duration of quick and slow eye-movements respectively are given.
TABLE I
| Subject | Film | Eye | Direction of the rotation. | Slow movements toward Subject's | Rapid movements toward Subject's | Average duration in seconds of slow movements | Average duration in seconds of rapid movements | |
|---|---|---|---|---|---|---|---|---|
| C | 1 | left | clockwise | right | left | .32 | .05 | |
| " | 2 | " | " | " | " | .36 | .06 | |
| " | 3 | right | anti-clock | left | right | .26 | .08 | |
| H | 1 | " | clockwise | right | left | .54 | .07 | |
| " | 2 | " | " | " | " | .45 | .07 | } |
| " | " | left | " | " | " | .45 | .07 | |
| " | 3 | " | " | " | " | .50 | .08 | } |
| " | " | right | " | " | " | .49 | .08 | |
| " | 4 | " | anti-clock | left | right | .49 | .07 | |
| " | 5 | left | clockwise | right | left | .53 | .06 | |
| Ta | 1 | right | " | " | " | .73 | .07 | |
| " | 2 | " | anti-clock | left | right | .48 | .10 | |
| Tu | 1 | " | clockwise | right | left | .50 | .06 | } |
| " | " | left | " | " | " | .49 | .07 | |
| " | 2 | " | " | " | " | .49 | .12 | } |
| " | " | right | " | " | " | .49 | .12 | |
| " | 3 | left | " | " | " | .40 | .07 | |
| " | 4 | right | anti-clock | left | right | .58 | .08 | |
| Av. | .48 | .08 |
Table I gives these averages for all the fourteen negatives. In four of these (H 2, H 3, Tu 1, Tu 2) simultaneous curves for both eyes were obtained. In every curve the slow eye-movements were in the same direction as the previous rotation; the rapid in the opposite direction. The very few single movements that are exceptions to this are noted under Table II. Had the photographs been taken during (instead of after) the rotation, the directions of rapid and slow movements would undoubtedly have been reversed. It is to be noted that when both eyes were recorded, their movements were generally identical, within the accuracy of measurement (one sixtieth of a second). There are a few exceptions to this. The averages of all slow and all rapid movements merely show that in general, and for that part of the nystagmus that was photographed, the slow eye-movements lasted six times as long as the rapid ones. This ratio varies considerably from one case to another, and at best throws little light on the whole nystagmiform series, since during the very first instants after the rotation the ratio of quick to slow movements would be less than one sixth, and at the very end of the series would be considerably more; this because toward the end the slow movements become much slower, while the rapid seem to change very little. The variations from case to case arise, at least partly, because in some cases the picture was taken more promptly, after the rotation stopped, than in others.