448 THE SENSORIAL FUNCTIONS. 



of being affected by luminous rays, and convey- 

 ing to the mind the sensation of light ; and this 

 part is the Retina, so named from the thin and 

 delicate membranous net-work, on which the 

 pulpy extremities of the optic nerves, establish- 

 ing an immediate communication between that 

 part and the brain, are expanded. 



If the eye were so constructed as to allow the 

 rays of light, which reach it from surrounding 

 objects, simply to impinge on the retina as they 

 are received, the only perception which they 

 could excite in the mind, would be a general 

 sensation of light, proportionate to the total 

 quantity which is sent to the organ from the 

 whole of the opposite hemisphere. This, how- 

 ever, does not properly constitute Vision ; for in 

 order that the presence of a particular object in 

 its real direction and position with respect to us, 

 may be recognised, it is necessary that the light, 

 which comes from it, and that light alone, should 

 produce its impression exclusively on some parti- 

 cular 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 actions would interfere with 

 one another, and only a confused impression 

 would result. The objects in a room, for ex- 

 ample, 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 



