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 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, 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, 

 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 



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