172 



SATURDAY LECTURES. 



could.) The adaptation or accommodation of the eye, there- 

 fore, for distinct vision at different distances is affected by 

 the other method ; that is, by a change in the curvature of 

 one of its two refracting surfaces. It has been abundantly 

 demonstrated by observation that this change is effected by 

 an increase in the curvature of the crystalline lens, brought 

 about by the action of a muscle situated on the interior of 

 the eye called the ciliary muscle, (Cm, Fig. 2.) By the con- 

 traction of this muscle the lens is rendered more convex, 

 and objects lying nearer the eye have their objects formed 

 more clearly on the retina. A reference to the diagram 

 (Fig. 3) will show how this is accomplished. When the ob- 

 ject is situated at A the image is formed on the retina, R, at 

 a, when it is at B it is formed t)ehind the retina at b. In 

 order, therefore, when the object is at B, to have its image 

 at a, the lens, L, must be made more convex, as shown by 

 the dotted line. 



Fig. 3- 



It is this failure of the power of accommodation or adap- 

 tation of the eyes to objects close at hand wliich constitutes 

 presbyopia or old eyes. With ffdvancing age the ciliary 

 muscle becomes stiff and less powerful, and the lens becomes 

 harder and alters its shape less readily, and the consequence 

 is that near vision is rendered less perfect. This is reme- 

 died by placing in front of the eye a lens which represents 

 the power that has been lost by the crystalline lens. Quite 

 the contrary of the generally accepted belief the eye does 

 not become materially flatter with age. 



We have now the image of the external object pictured 

 clearly and distinctly upon the expansion of the optic nerve 

 at the bottom of the eye, and with this the whole of the 

 function of the eye as an optical instrument is ended. 



