SENSE OF SIGHT. 859 



forming an image and that of a simple lens is evident in the fact that 

 with the normal eye an object may be seen with equal distinctness 

 whether at a great distance or closely approached to the eye. This 

 indicates that there must be some mechanism in the eye by which the 

 focal length of the refracting media can be altered. If the finger be 

 held a short distance before the eye, the other being closed, it may 

 be distinguished with the greatest distinctness. If the finger remain 

 in the same position, and the eye now be fixed on some more distant 

 object, the finger is seen indistinctly, so that at will we may focus our 

 eyes on either the near or the far object and either may be distinctly 

 seen: yet when the eye is arranged for near objects far objects are seen 

 indistinctly, and the reverse. Such an adjustment of the refracting 

 powers of the eye is termed accommodation. It was for a time thought 

 that the accommodation of the e}'e was accomplished by approaching or 

 withdrawing the receiving surface, the retina, to or from the lens. It 



FIG. 381. DIAGRAM ILLUSTRATING THE FORMATION OF AN IMAGE ON THE 

 RETINA. (Yeo.) 



The rays from the point, a, passing through the cornea, lens, etc., are collected in the retina at 6. Thosa 

 from a' meet at b', and thus the lower point becomes the upper. 



has, however, been shown that this is not the case, and that accommo- 

 dation is accomplished by changes in the curvature of the cr3 r stalline lens. 

 Referring again to our illustration of an object, a convex lens, and a 

 screen, as has been stated, if we determine the focal length of the lens 

 or the point at which a distinct image will be formed upon the screen and 

 then approach the object to the lens, the screen not being removed from 

 its position, the image will be indistinct. If, now, we remove the object, 

 the screen remaining unmoved, and substitute for the lens originally 

 used one of a greater degree of curvature, the degree of refraction will 

 evidently be greater and the focal length shorter, so that the rays from 

 the object in the near position, being more diverging, may be brought to 

 a focus at a point corresponding to that of the less diverging rays from 

 the farther-removed object. So, again, if the point of distinct image be 

 determined and the object farther removed, the screen would then have 

 to be approached to the lens in order to obtain a distinct image. In this 



