IMAGES ON THE RETINA. 



whose refracting powers would correspond with each, of the 

 humours. 



15. That this phenomenon is actually produced, may he ren- 

 dered experimentally manifest hy taking the eye-ball of an ox 

 recently killed, and dissecting the posterior part, so as to lay hare 

 the choroid. If the eye thus prepared he fixed in an aperture in a 

 screen, and a candle he placed before it at a distance of eighteen 

 or twenty inches, an inverted image of the candle will be seen 

 through the retina, as if it were produced upon ground glass or 

 oiled paper. 



16. It appears, then, that the immediate cause of vision, and 

 the immediate object of perception, is the image thus pro- 

 duced by means of the refracting powers of the humours of 

 the eye. 



17. In order, therefore, to perfect vision, the following conditions 

 must be fulfilled : 



1. The image must be perfectly distinct. 



2. It must have sufficient magnitude, 



3. It must be sufficiently illuminated. 



4. It must continue on the retina for a sufficient length of 

 time. 



Let us examine the. circumstances which affect these conditions. 



18. 1. DISTINCTNESS OP THE IMAGE. 



The image formed on the retina will be distinct or not, according 

 as the pencils of rays proceeding from each point of the object 

 placed before the eye, are brought to an exact focus on the retina 

 or not. If they be not brought to an exact focus on the retina, 

 their focus will be a point beyond the retina, or within it. In 

 either case, the rays proceeding from any part of the object, 

 instead of forming a corresponding point on the retina, will form 

 a spot of greater or less magnitude, according to the distance of 

 the focus of the pencil from the retina, and the assemblage of 

 such luminous spots will form a confused picture of the object. 

 This deviation of the foci of the pencils from the retina is caused 

 by the refracting powers of the eye being either too feeble or too 

 strong. If the refracting powers be too feeble, the rays are inter- 

 cepted by the retina before they are brought to a focus ; if they 

 be too strong, they are brought to a focus before they arrive at 

 the retina. 



19. The objects of vision may be distributed into two classes, 

 in relation to the refracting powers of the eye : 1st, Those which 

 are at so great a distance from the eye, that the pencils pro- 

 ceeding from them may be regarded as consisting of parallel 

 rays ; 2ndly, Those which are so near that their rays have sensible 

 divergence. 



55 



