THE OPTICAL DEFECTS OF THE EYE. IS* 



lens, they are brought to a focus one inch behind the lens, but if an 

 object, for instance the flame of a lamp, be brought to within four 

 inches of the lens, we know that the focus will fall farther than one 

 inch behind the lens. If we wish to receive the inverted image of the 

 lamp upon a screen, the screen must be held one inch and a third 

 behind the lens. 



Now when an object is brought to within, say four inches of the eye, 

 it has no power to move the retina backwards to receive the image 

 that would be formed behind that membrane, but, what answers the 

 same purpose, it has the property of so far increasing its refractive 

 power, as to be able not only to render parallel, these diverging rays, 

 but also to focus them upon the retina. This increase in the power 

 of the eye, is equal to the addition of a 4 inch lens in front of an eye 

 that has its " accommodation " paralysed, as a 4 inch lens renders 

 rays parallel that diverge from objects four inches distant. 



Fig. 8 represents the section of a normal eye. When it is accom- 

 modated for distant objects parallel rays P, P, are focussed upon the 

 retina at F, while diverging rays from O, would form a focus at/c?. 

 When, however, the eye is accommodated for the near object O, these 

 diverging rays are focussed upon the retina at F. 



The manner in which this increase in the refractive power of the eye 

 is effected is still a disputed point. Most physiologists however are 

 now inclined to the theory that it is caused by an increase in the cur- 

 vature, — a thickening from before backwards, of the crystaline lens.* 



* The accommodotion of the eye was at one time believed to be produced by 

 the external muscles, but it is now ascertained that the accommodation can remain 

 perfect with all the external muscles paralysed. 



The iris was thought, by others, to have the power of increasing the refractive 

 power of the ej e, but it was proved by a case that occurred in Dr. Von Graefe'a 

 practice that accommodation can still be effected with entire absence of the iris. 



Helmholtz and Cramer have proved by means of the opthalmometre, that when 

 the eye is accommodated for a near object it undergoes the following changes : — 



