ADAPTATION TO VARIOUS DISTANCES. 651 



obvious that the effect might be produced in either of two 

 ways, viz., by altering the convexity or density, and thus 

 the refracting power, either of the cornea or lens ; or, by 

 changing the position either of the retina or of the lens, so 

 that whether the object viewed be near or distant, and the 

 focal distance thus increased or diminished, the focal point 

 to which the rays are converged by the lens may always be 

 at the place occupied by the retina. The amount of either 

 of these changes required in even the widest range of 

 vision, is extremely small. For, from the refractive powers 

 of the media of the eye, it has been calculated by Olbers, 

 that the difference between the focal distances of the 

 images of an object at such a distance that the rays are 

 parallel, and of one at the distance of four inches, is only 

 about 0.143 f an inch. On this calculation, the change 

 in the distance of the retina from the lens required for 

 vision at all distances, supposing the cornea and lens to 

 maintain the same form, would not be more than about 

 one line. 



It is now almost universally believed that Helmholtz is 

 right in his statement that the immediate cause of the 

 adaptation of the eye for objects at different distances is a 

 varying shape of the lens, its front surface becoming more 

 or less convex, according to the distance of the object 

 looked at. The nearer the object, the more convex does 

 the front surface of the lens become, and vice versd ; the 

 back surface taking little or no share in the production of 

 the effect required. Of course, the lens has no inherent 

 power of contraction, and therefore its changes of outline 

 must be produced by some power from without ; and there 

 seems no reason to doubt that this power is supplied by 

 the ciliary muscle. The exact manner, however, in which, 

 by its contraction, the ciliary muscle effects a change in 

 the shape of the crystalline lens is doubtful. The most 

 probable explanation of the phenomenon, however, is that 

 in adapting the eye for viewing near objects the ciliary 



