1912.] Distance of Centres of Rotation of the Two Eyes. 55 



geometrical construction, or, assuming parallelism between the base line EL 

 and the scale SS, V may be calculated from the following data : — 



Let the distance KM = a, ps = b, qn = c, KL = V. 

 Then from fig. 2 we have 



b : V = e-a : a-V, or ab-bV = (c-a) V ; 

 that is V = ab/(b+c—a). 



Since a is constant for a given instrument, the values of V may in practice 

 be obtained from a table computed in terms of b and c. 



Further, it may be pointed out that in all the methods hitherto employed 

 for measuring the distance between the centres, it is essential that the 

 subject should have binocular vision, in order to enable him to fix an object 

 with both eyes simultaneously. By either of the methods just described 

 it is possible to measure the distance between the centres of persons who 

 cannot fuse the images of the two eyes, or even, provided the vision of each 

 eye be sufficient, to measure this distance in the case of persons who squint. 

 Moreover, it is not necessary that the vision should be distinct so long as 

 the eye can distinguish the index. Good measurements can be made even 

 though the definition is not sufficient to enable large print to be read. 

 Various methods may be used to increase accuracy of scale reading, such as 

 verniers and multiplying levers. 



