ON INSTRUMENTS AND THEIR USE. 17 



8 to 10 inches. As we move farther away from 

 the object, although diminished in size, it may be 

 seen more easily, and with less effort. 



It would appear, then, that all objects are ren- 

 dered apparently larger, as they continue to 

 approach the eye, but a limit is soon found to this, 

 as at a distance of six inches distinct and easy 

 vision is not possible (except in very abnormal 

 cases). 



The reason of this is well-known the anterior 

 focal point of a convex lens when shortened 

 lengthens the posterior conjugate focus, so that 

 when an object is brought too near the eye the 

 image of it is projected behind the retina, and the 

 crystalline lens cannot accommodate itself to such 

 extremes. But we know that objects can be seen 

 distinctly at great distances apart, and it may be 

 useful to demonstrate how this is brought about. 



The real mechanism of accommodation has been 

 much disputed, but the results, as observed, are, 

 that the curvatures of the crystalline lens are 

 altered as the observer adapts his eye to near or 

 remote vision ; increase of curvature, of course, 

 shortening the focal length of the crystalline lens, 

 and being better adapted for near vision, while the 

 shallower curve is necessary for the distant view of 

 remote objects. Helmholtz has shown that the 

 radius of curvature of the anterior surface of the 

 crystalline lens may be varied by means of the 

 muscular arrangement, from 6 to 10 millimetres. 



We may now cast another glance at the iris. 



This apparatus is really a continuation of the 



choroid tunic which lies between the sclerotica and 



the retina : it ends in front, in what are called 



2 



