LABORATORY MANUAL OF PHYSIOLOGY. 



meant the union, in one single impression, of images received 

 simultaneously on both retinae. The external ocular muscles 

 maintain the visual axes parallel, so that impressions of an object 

 fall on correspondingly identical points of both retinae (see Fig. 



44)- 



What happens when paralysis of one ocular muscle destroys 



balance ? Show by diagram. 



CONVERGENCE. Let the subject fix his vision upon an object 

 within an arm's length, and then upon some object more than six 

 metres distant but in the same line of vision. 



What change takes place in relation of the visual axes to each 

 other? 



Hold an object one metre distant, directly in front of the subject. 

 Move it directly toward the subject's eyes. Note the convergence 

 of the visual axes. 



What change takes place in the pupil? What muscles are in- 

 volved in the act? Recall the innervation of the pupil and the 

 internal recti. 



OPTICAL DEFECTS. 



The eye is not a perfect optical instrument, since it is not exactly 

 centred and possesses in small degree chromatic and spherical 

 aberration. 



By chromatic aberration is meant that different rays of the 

 spectrum are bent to different degrees. For instance, violet rays, 

 which are more refrangible than red, have their focus near to the 

 lens. In the manufacture of optical instruments, this is overcome 

 by combining a convex with a plano-concave lens. Practically the 

 same arrangement exists in the eye, and this, combined with the 

 rapid accommodative ability of the lens, makes chromatic aberra- 

 tion a negligible quantity. 



By spherical aberration is meant that rays of light which traverse 

 the periphery of a lens are brought to a focus sooner than those 

 which pass nearer the centre. The iris corrects this defect by 

 acting as a diaphragm, shutting off the peripheral rays. 



[186] 



