BINOCULAR VISION 



203 



We have seen tliat the I'linctions of acconmiodation and con- 

 vergence are within hnnts co-ordinated, i.e. there is a movement 

 inwards of each eye of 1 metre angle for each dioptre of accom- 

 modation. Now when the eyes are dissociated, and this may 

 easily be done by holding a card sufficiently near one eye to prevent 

 vision of the object by it and yet far enough aw^ay to permit of 

 observation of that eye, and an object brought gradually from the 

 far point (6 metres away) to the near point (7-17 em.), accom- 

 modation and convergence do not keep pace with one another. 

 The occluded eye will deviate about 3° or 4° outwards, i.e. at 

 the near point all emmetropes are exophoric. As the object is 

 taken further away from the eye the exophoric deviation of the 

 dissociated eye tends to become less, and 

 at a certain point, usually about 30-50 cm. 

 away, no deviation is noticeable ; that 

 is, the extra-ocular muscles are perfectly 

 balanced. The mechanism is, therefore, 

 orthophoric. The latent deviation, or 

 heterophoria, may be in a variety of 

 directions, depending on which of the 

 extra-ocular muscles are unbalanced. The 

 directions may, however, for purposes of 

 description, be resolved into four com- 

 ponents pulling at right angles to the 

 visual axis, e.g. upwards or downwards ; 

 inwards or outwards. The former two are 

 deviations due to unbalanced action round 

 a horizontal axis, and are termed hyper- 

 phoria or latent vertical deviations. 

 Latent convergence is called esophoria, and latent divergence is 

 exophoria. About three-quarters of the people with emmetropic 

 eyes are esophoric, less than one-fifth are exophoric, and only 

 about one in twenty is orthophoric. The value of the deviation 

 is generally given as so many dioptres — the power of the lens 

 required to correct it. The type of lens used depends on whether 

 the object is to be " brought in " or " taken outwards." Con- 

 sequently to correct heterophoria decentred lenses are used, 

 convex lenses displaced against the deviation, and concave 

 lenses with the deviation found by tests. 



3. Divergence. In normal circumstances the visual axes never 

 diverge, for they are in parallel adjustment for objects at infinity. 

 Divergence can, however, be brought about artificially. Thus if 

 we interpose prisms to render the rays divergent we can produce 

 a corresponding divergence of the visual axes. 



<a 



Fig. 



-Artificial divergence of 

 visual axes. 



