166 SENSORIAL FUNCTIONS. 



same time is rendered more indistinct, because then the 

 rays interfere, and mingle with each other 



501. The only method by which distinctness of the image 

 and increased illumination can be obtained, is by collect- 

 ing into one point a great number of rays, proceeding 

 from the corresponding points of the object to be repre- 

 sented. This intention is answered by the use of a 

 short tube containing a double-convex lens, such as is 

 represented by Fig. 101, mserted in the aperture. By this 

 instrument, the rays are collected by refraction, and con- 

 centrated so as to present a perfectly-defined, and highly 

 illuminated picture. 



502. 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. 



503. 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. 



504. This fact, together with the known effect of the 

 convex 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 con- 

 sidered 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, {Jiey 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 



