BINOCULAR VISION 225 



Neither of these defects interferes appreciably with the accuracy 

 of vision. 



(iii) Slight degrees of astigmatism are almost always present. 



(iv) Chromatic aberration is uncompensated optically. The 

 retina, however, is less sensitive to the extremes of the spectrum, 

 consequently the slight halo of red rays focussed behind and of 

 violet focused in front produces no sensation. 



(v) Spherical aberration is provided for by the iris which cuts 

 off the peripheral rays. This is particularly noticeable in accom- 

 modation where in order to exclude the more divergent rays the 

 pupil contracts. 



Binocular Vision. 



1. The Visual Field. When one eye is closed and the other fixed 

 on a certain point the whole range of objects which can be seen 

 without moving the eye or the head is called the visual field. 

 The angle subtended by these objects is called the visual angle. 

 With both eyes open and fixed we command a greater range. 

 (For the mapping out of the visual field and measurement of the 

 visual angle see pages 425 and 426.) 



2. Focusing of objects within the far point. We have seen that 

 in order to focus near objects we employ accommodation. 

 This is always accompanied by a movement of the eyeballs, 

 causing the visual axes to converge from their parallel resting 

 position towards the middle line. Such a movement is necessary 

 in order that the images in both eyes may fall on the fovea centralis 

 (Fig. 52). If this mechanism is defective, strabismus or squint 

 results. 1 ^ The amount of convergence varies inversely as the 

 distance of the object. This may be proved as follows (Fig. 52) : 



OP A O'P' OP 



Sin OEP OP O'E O'E 



Sin '' OP OE ' 



Since OEP and O'EP' are small, sin OEP =OEP and sin O'EP' 



OEP O'E 



Hence = . 



O'EP' OE 



The power of convergence is rather less than that of accommoda- 

 tion, for we can focus an object with one eye at a slightly shorter 

 distance than when we use both eyes. 



B.B. 15 



