VISUAL ANALOGUES 177 
closing them, the composite image is analysed alternately 
into its component parts, and thus we are enabled to 
read better with eyes shut than open. 
This period of alternation is modified by age and 
by the condition of the eye. It is, generally speaking, 
shorter in youth. I have seen it vary in different indi- 
viduals from 1” to 10” or more. About 4” is the most 
usual. With the same individual, again, the period is 
somewhat modified by previous conditions of rest or 
activity. Very early in the morning, after sleep, it is 
at its shortest. I give belowa set of readings given by 
an observer : 
Period | Period 
PNM hada celeaicteass sas ou GrP iM ae aeons pA 
EETLOON .i5.00'ste onto sis ¥ive 4” | Di bay) | foaervseawat-oaeas 56" 
SS TERT ae Ry" ella Meas. esate eee 6:5!” 
Again, if one eye be cooled and the other warmed, 
the retinal oscillation in one eye is quicker than in the 
other. The quicker oscillation overtakes the slower, 
and we obtain the curious phenomenon of ‘¢ visual 
beats.’ 
After-images and their revival—tIn the experiment 
with the stereoscope and the design of the cross, the 
after-images of the cross seen with the eyes closed are 
at first very distinct—so distinct that any unevenness 
at the edges of the slanting cuts in the design can be 
distinctly made out. There can thus be no doubt of 
the ‘objective’ nature of the strain impression on the 
retina, which on the cessation of direct stimulus of 
light gives rise to after-oscillation with the concomitant 
visual recurrence. This recurrence may therefore be 
taken as a proof of the physical strain produced on the 
N 
