VISION. 449 



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 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 accomplishmg 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 oi 

 all the rays, proceeding from the respective 

 points of the object to be viewed, on separate 

 points of the retina, and obtain likewise the ex- 

 clusion 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 delineation, in miniature, of the external 

 scene. 



As it is a fundamental law in optics that the 



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