Distance given by Binocular Vision. 315 



way, unite figures whose distance exceeds 2| inches, the in- 

 terval between the eyes. Transparent patterns for these ex- 

 periments may be cutout of stiff" card- paper, or thin plates of 

 metal, or they may be made of paper pasted upon large panes 

 of glass. Experiments may be macle with trellis-work, or 

 with windows composed of small squares or lozenges; but the 

 readiest pattern is the cane bottom of a chair, and I have per- 

 formed my experiments by simply placing such a chair upon 

 a high table, with its cane bottom in a vertical position. The 

 distance of the centres of the eight-sided open figures in the 

 direction of the width or depth of the chair, varies in different 

 patterns from 0*54 to 0*76 of an inch. In order to simplify 

 the calculations, we shall take the distance at 0*5, or half an 

 inch. Then let 



D=12 inches be the distance of the pattern from the eyes. 

 d—0'5 the distance of the centres of the similar figures. 

 + A = distance of suspended image from, and in front of, the 



pattern. 

 — A'=distance of suspended image from, and behind, the 



pattern. 

 C=2*5 the distance between the eyes. 

 Then we shall have 



+ A=: 



Tid 



and — A' = 



Drf 



Hen( 



C + ^""" C-d' 



D — A = distance of suspended image from the eye, and in 



front of the pattern, and 

 D + A' = its distance from the eye, and behind the pattern. 



From these formulae we have computed the following table, 

 adapted to similar figures, whose centres are distant i an inch, 

 1, li, 2 and 2^ inches; but in reference to the positive values 

 of A and D, we may consider them as feet, 0'5 being in that 

 case =6 inches. 



Taking the case where D is 12 inches, and uniting the two 

 nearest openings where d is 0"5, let M N (fig. 4) be a section 

 of the transparent pattern, L, II the left and right eyes, Lar/, 

 \^bc lines drawn through the centres of twoof the oj)en figures 

 ab, and libd^ lice lines drawn through the centre of i and c, 



