292 OUTPOSTS OF THE INTELLIGENCE SERVICE 



axis counteract each other, the remaining two act as a couple 

 and reinforce each other. The result of the combined action is 

 to move the cornea vertically upwards. For movement upwards 

 and inwards, for instance, the co-operation of a third muscle, the 

 internal rectus, is required. The rectus inferior and the obliquus 

 superior combine in an exactly similar manner to move the eyeball 

 downwards (Fig. 75), 



Binocular Vision. 



1. 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. 76). If this 

 mechanism is defective, strabismus or 

 squint results. The amount of con- 

 vergence varies inversely as the distance 

 of the object. The power of conver- 

 gence is rather less than that of accom- 

 modation, for we can focus an object 

 with one eye at a slightly shorter distance 

 than when we use both eyes. 



It is only in the emmetropic eye that 

 convergence and accommodation corre- 

 spond exactly in amount. A hyper- 

 metropic person viewing a near object 

 may require to use, say 4 dioptres of 

 accommodation, whereas an emmetropic 

 person uses only 2 dioptres, but both 

 employ the same amount of convergence. Similarly an emme- 

 trope may focus a near object, and if concave lenses are placed 

 in front of his eyes he can continue to see the object plainly 

 by increasing his acconnnodation sufficiently to neutralise their 

 effect. The object still remains at the same distance, i.e. the 

 convergence does not alter. Hence we see that accommodation 

 and convergence are to some extent independent. 



2. Heterophoria. In strabismus the deviation of the two visual 

 axes is manifest, but the majority of people, with otherwise normal 

 eyes, have a latent squint, only exhibited as an actual deviation 

 when the eyes are dissociated by Maddox rods or other apparatus. 



riG. 76. — Convergence. OandO' 

 are the far and near points respec- 

 tively, and OEF and O'EP the 

 angles of convergence. 



