170 SENSORIAL FUNCTIONS. 



same time is rendered more indistinct, because then the 

 rays interfere, and mingle with each other. 



The only method by which distinctness of the image and 

 increased illumination can be obtained, is by collecting 

 into one point a great number of rays, proceeding from 

 the corresponding points of the object to be represented. 

 This intention is answered by the use of a short tube 

 containing a double convex lens, such as is represented 

 by Fig. 101, inserted into the aperture. By this instru- 

 ment, the rays are collected by refraction, and concen- 

 trated so as to present a perfectly defined and highly 

 illuminated picture. 



Now these illustrations, and principles, are exactly 

 those which apply to the mechanism, and use of the 

 human eye ; which, in all respects, is a camera obscura, 

 of the most perfect workmanship. The vitreous humor 

 is the space occupied by the darkened chamber ; the 

 pupil is the aperture through which the light is admitted ; 

 the crystalline humor is the double convex lens, by which 

 the rays of light are collected and concentrated ; and 

 the retina is the screen, on which the picture is painted, 

 in an inverted position, by the crossing of the rays. 



If the eye of an animal be prepared by cutting away 

 the sclerotic coat, and optic nerve on the back side, so 

 as to make this part thin and transparent, objects seen 

 through it appear in the form of an inverted picture on 

 the retina. 



This fact, together with the known effect of the con- 

 vex lens to invert the images of objects, is sufficient 

 to prove, beyond all doubt, that the image is inverted on 

 the retina. This might perhaps, at first thought, be 

 considered as an imperfection in the eye, but we find 

 that nature always attains her objects by the most direct 

 and simple means. Another lens placed in the focus of 

 the crystalline, would have corrected this inversion ; and 

 we find that finite mechanics resort to this method in 

 the construction of terrestrial telescopes ; that is, they 

 add one more glass on purpose to correct the inversion 

 of objects. But it is well known that this additional 

 glass always proves an imperfection in the instrument 

 on other accounts, since every glass, however perfect it 

 may appear, still intercepts a portion of the light. In 



