402 THE SENSORIAL FUNCTIONS. 



paper laid on the floor ; but this light, being spread 

 equally over every part of the surface of the paper, 

 furnishes no means of distinguishing the sources 

 from which each portion of the light has proceeded; 

 or, in other words, of recognising the respective 

 figures, situations, and colours of the objects them- 

 selves. We shall now proceed to consider the 

 modifications to be introduced into the structure of 

 the organ, in order to accomplish these purposes. 



§ 2. Modes of accomplishing the Objects of Vision. 



Let us suppose that it were proposed to us as a 

 problem to invent an apparatus, by which, availing 

 ourselves of the known properties of light, we might 

 procure the concentration of all the rays, proceed- 

 ing from the respective points of the object to be 

 viewed, on separate points of the retina, and obtain 

 likewise the exclusion of all other rays ; and also 

 to contrive that the points of the retina, so illumi- 

 nated, should have the same relative situations 

 among one another, which the corresponding 

 points of the surrounding objects have in nature. 

 In other words, let us suppose ourselves called 

 upon to devise a method of forming on the retina 

 a faithful delineation, in miniature, of the external 

 scene. 



As it is a fundamental law in optics that the rays 

 of light, while they are transmitted through the 

 same medium, proceed in straight lines, the sim- 

 plest mode of accomplishing the proposed end 

 would be to admit into the eye, and convey to each 

 particular point of the retina, only a single ray 



I 



