CHAP, m.] SIGHT. 851 



of effort often spoken of by myopic persons as being felt when 

 they attempt to see things at or beyond the far limit of their 

 range seems to arise from a movement of the eyelids, and not 

 from any internal changes taking place in the eye. 



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. 144 A, a) 

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

 a second less bright, 5, by the anterior surface of the lens, and a 



