318 THE SENSORIAL FUNCTIONS. 



on some particular part of the retina; it being evident that 

 if the light, coming from any other object, were allowed to 

 act, together with the former, on the same part, the two ac- 

 tions would interfere with one another, and only a confused 

 impression would result. The objects in a room, for exam- 

 ple, are all throwing light on a sheet of paper laid on the 

 floor; but this light, being spread equally over every part of 

 the surface of the paper, furnishes no means of distinguish- 

 ing 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 themselves. 

 We shall now proceed to consider the modifications to be 

 introduced into the structure of the organ, in order to attain 

 these objects. 



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 concentra- 

 tion of all the rays, proceeding 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 illuminated, shall 

 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 deli- 

 neation, 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, pro- 

 ceed in straight lines, the simplest mode of accomplishing 

 the proposed end would be to admit into the eye, and con- 

 vey to each particular point of the retina, only a single ray 

 proceeding directly from that part of the object which is to 

 be depicted on it, and to exclude all other rays. For car- 



