18 ACCOMMODATION. [BOOK in. 



What then are the changes which take place in the eye, when 

 we accommodate for near objects? It might be thought, and 

 indeed once was thought, that the curvature of the cornea was 

 changed, becoming more convex, with a shorter radius of curvature, 

 for near objects. This is disproved by the fact that accommodation 

 takes place as usual when the eye (and head) is immersed in water. 

 Since the refractive powers of aqueous humour and water are very 

 nearly alike, the cornea, with its parallel surfaces, placed between 

 these two fluids, can have little or no effect on the direction of 

 the rays passing through it when the eye is immersed in water. 

 Moreover we have it in our power to detect any change in the 

 curvature of the cornea which may take place. If a luminous 

 body such as a candle be held in front of a convex surface like 

 the cornea an image of the body is seen reflected from the surface ; 

 and, with the body at a certain distance, the image will be of a 

 certain size. If now the curvature of the surface be increased, 

 if the surface be made more convex, the image will diminish in 

 size ; if the curvature of the surface be diminished the image will 

 increase in size. Indeed by measuring carefully the changes in the 

 size of the image we may determine the amount of change in the 

 curvature of the surface. And accurate measurements of the 

 dimensions of an image on the cornea have shewn that these 

 undergo no change during accommodation, and that therefore 

 the curvature of the cornea is not altered. Nor is there any 

 change in the form of the bulb ; for any variation in this would 

 necessarily produce an alteration in the curvature of the cornea, 

 and pressure on the bulb would act injuriously by rendering the 

 retina anaemic and so less sensitive. In fact, there are only two 

 changes of importance which can be ascertained to take place in 

 the eye during accommodation for near objects. 



One is that the pupil contracts. When we look at near objects, 

 the pupil becomes small ; when we turn to distant objects, it 

 dilates. This however cannot have more than an indirect influence 

 on the formation of the image ; the chief use of the contraction of 

 the pupil in accommodation for near objects is to cut off the more 

 divergent circumferential rays of light. 



The other and really efficient change is that the anterior 

 surface of the lens becomes more convex. If a light be held 

 before the eye, three reflected images may, with care and under 

 proper precautions, be seen by a bystander: one (Fig. 139 A, a) 

 a very bright one caused by the anterior surface of the cornea, 

 a second less bright, b, by the anterior surface of the lens, and a 

 third very dim, c, by the posterior surface of the lens ; when the 

 images are those of an object, such as the flame of a candle, in 

 which a top and bottom can be recognised, the two former images 

 are seen to be erect, but the third inverted. When the eye is 

 accommodated for near objects, no change is observed in the first, 

 and none, or a very insignificant one, in the third of these images ; 



