now WK SEE. 1 / 1 



standpoint. This is, however, a <:;rcat error. Considered as 

 an optical instrument, wliose function it is to form the clear- 

 est possible imaj;e, it is defective in almost all its parts, and 

 does not hoggin to aj)proach the best work of the same kind 

 as done by man. Ilelmnolt/., the jjijreat German physicist, 

 lias saicl that if an optician were to send him an instrument 

 so badly constructed as tlie eye, he would return it to him 

 with a severe reprimand for liis carelessness. But we must 

 bear in mind that the eye is an or^an of sense as well as an 

 optical hislrument, and with all its defects, is caj)al)l(! of ren- 

 dering us most efficient service. 



As I have said, the function of the e^'e proi)er is to form 

 elcarly-delined inverted images on the retina. The impres- 

 sion or changes produced b^'this image are convej'ed to the 

 brain by means of the optic nerve, and are there converted 

 into a .sensation. A confused image will cause a confused 

 sensation, and a clearly-defined image a corresponding sen- 

 sation. It is, therefore, of the highest importance that we 

 always haveretinal images whose outlines are sharp and dis- 

 tinct. This is very easily accompushed when the object 

 and the retina occupy certain rekitive positions to each 

 other. It is one of the laws of optics that when the object 

 is at a certain di.stance (within specified limits) from the 

 lens the image will be formed at a certain distance from 

 it on the other side. If the distance of the object is changed, 

 the distance of the image from the lens will also be altercl. 



Now. in using the eyes, give look at objects within a few 

 inches of us and also at those at immense distances. The 

 range of acconnnodation, as it is called, extends from four 

 inches from the eye to infinity. If we are to have clearly 

 defined images of all objects in this extensive range some 

 <hange must take place in the eye corresponding to t.iie 

 change in the distance of the object. Now this can be ac- 

 complished in ohe of two ways — either by a change in the 

 distance between the lens and the retina, or by an increa.se 

 in the curvature of the refracting surfaces. The first of 

 these cannot be readily accomplished, because the eye can- 

 not be made longer, (though it was once supposed that it 



