176 RESPONSE IN THE LIVING AND NON-LIVING 



slanting cuts at a suitable distance from each other. 

 One cut, E, 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 nice 



, , versa. The alternate fluctuations 



become far more conspicuous when 

 the eyes are closed ; the pure oscil- 

 latory after-effects are then obtained 

 in a most vivid manner. After 

 looking through the stereoscope for 

 ten seconds or more, the eyes are 

 ' DESIGN E " closed. The first effect observed is 

 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 



