THE SENSE OF VISION. 753 



sure produced by the eye-muscles being assumed to occur. This lengthening 

 would, in the case of a normal eye accommodating itself for an object at a 

 distance of 15 centimeters, amount to not less than 2 millimeters a change 

 which could hardly be brought about by the action of any muscles connected 

 with the eye. Moreover, accommodation changes can be observed upon elec- 

 trical stimulation of the excised eye. Its mechanism must, therefore, lie within 

 the eye itself. As for the second of these methods, there is no conceivable way 

 by which a change in the index of refraction of the media can be effected, and 

 we are thus forced to the conclusion that accommodation is brought about by 

 a change in the curvature of the refracting surfaces i. e. by a method quite 

 different from any which is employed in optical instruments of human con- 

 struction. Now, by measuring the curvature of the cornea of a person who 

 looks alternately at near and distant objects it has been shown that the cornea 

 undergoes no change of form in the act of accommodation. By a process of 

 exclusion, therefore, the lens is indicated as the essential organ in this function 

 of the eye, and, in fact, the complicated structure and connections of the lens 

 at once suggest the thought that it is in the surfaces of this portion of the eye 

 that the necessary changes take place. Indeed, from a teleological point of 

 view the lens would seem somewhat superfluous if it were not important to 

 have a transparent refracting body of variable form in the eye, for the amount 

 of refraction which takes place in the lens could be produced by a slightly 

 increased curvature of the cornea. Now, the changes of curvature which occur 

 in the surfaces of the lens when the eye is directed to distant and near objects 

 alternately can be actually observed and measured with considerable accuracy. 

 For this purpose the changes in the form, size, and position of the images of 

 brilliant objects reflected in these two surfaces are studied. If a candle is held 

 in a dark room on a level with and about 50 centimeters away from the eye in 

 which the accommodation is to be studied, an observer, so placed that his own 

 axis of vision makes about the same angle (15 20) with that of the ob- 

 served eye that is made by a line joining the observed eye and the candle, will 

 readily see a small upright image of the candle reflected in the cornea of the 

 observed eye. Near this and within the outline of the pupil are two other 

 images of the candle, which, though much less easily seen than the corneal 

 image, can usually be made out by a proper adjustment of the light. The 

 first of these is a large faint upright image reflected from the anterior surface 

 of the lens, and the second is a small inverted image reflected from the pos- 

 terior surface of the lens. It will be observed that the size of these images 

 varies with the radius of curvature of the three reflecting surfaces as given on 

 p. 749. The relative size and position of these images having been recog- 

 nized while the eye is at rest i. e. is accommodated for distance let the 

 person who is under observation be now requested to direct his eye to a near 

 object lying in the same direction. When this is done the corneal image and 

 that reflected from the posterior surface of the lens will remain unchanged, 1 



1 A very slight diminution in size may sometimes be observed in the image reflected from 

 the posterior surface of the lens. 

 48 



