176 RESPONSE IN THE LIVING AND NON-LIVING 
slanting cuts at a suitable distance from each other. 
One cut, R, slants to the right, and the other, L, to the 
left (see fig. 111). When the design is looked at 
through the stereoscope, the right eye will see, say R, 
and the left L, the two images will appear superimposed, 
and we see an inclined cross. When the stereoscope is 
turned towards the sky, and the cross looked at steadily 
for some time, it will be found, owing to the alternation 
already referred to, that while one arm of the cross 
begins to be dim, the other becomes bright, and vice 
versa. The alternate fluctuations 
become far more conspicuous when 
R the eyes are closed; the pure oscil- 
latory after-effects are then obtamed 
in a most vivid manner. After 
looking through the stereoscope for 
ten seconds or more, the eyes are 
Fie. TL SepEno- closed. The first effect observed is 
scopic DESIGN 
one of darkness, due to the rebound. 
Then one luminous arm of the cross first projects 
aslant the dark field, and then slowly disappears, after 
which the second (perceived by the other eye) shoots 
out suddenly in a direction athwart the first. This 
alternation proceeds for a long time, and produces the 
curious effect of two luminous blades crossing and 
recrossing each other. 
Another method of bringing out the phenomenon of 
alternation in a still more striking manner is to look at 
two different sets of writing, with the two eyes. The 
resultant effect is a blurr, due to superposition, and the 
inscription cannot be read with the eyes open. - But on 
