﻿of Binocular Vision. 135 



Those who are not accustomed to experiments of this kind 

 can probably most easily succeed as follows. If we look on the 

 floor and place the finger between the eye and the floor, the 

 finger will of course be seen double. Now move the finger up 

 or down until we find a place in which the two images of the 

 finger will exactly fall on contiguous figures of the pattern ; the 

 finger now indicates the position of the first plane. Now look 

 steadily at the finger instead of the floor until the image of the 

 floor rises to it and becomes distinct, then withdraw the finger. 

 To get the second plane, look again at the floor and raise the 

 finger until its images fall upon figures separated from one an- 

 other by an intervening figure and then look steadily at the 

 finger. The other planes may be obtained in a similar manner. 

 The position of the several planes may also be easily calcu- 

 lated — the data being the interocular line, the distance of the 

 object, and the interval from centre to centre of the figures. 

 Both by measurement and calculation I determined the planes 

 in the case of the carpet to be 21*5, 13*05, 9'37, and 7*3 

 inches respectively. In the case of the oil-cloth they were 11 '8, 

 6*54, and 4*5 inches. 



If the distance between the centres of contiguous figures 

 be less than the interocular line, then still other images may 

 be seen beyond the real object and very much enlarged. The 

 position of the eyes and the place of the image in this case also 

 is easily explained. If dab c (fig. 3) be the plane of the real 



Fig. 3. 



object, and the eyes I and II be directed toward contiguous 

 figures a and b but not crossed, then the image of a and b will 

 combine and be seen at a\ the intersection of the visual lines. 



