VISION. 401 



The sense of Vision is intended to convey to its 

 possessor a knowledge of the presence, situation, 

 and colour of external and distant objects, by 

 means of the light which those objects are con- 

 tinually sending off, either spontaneously, or by 

 reflection from other bodies. We know of only 

 one part of the nervous system so peculiarly organ- 

 ized as to be capable of being affected by luminous 

 rays, and conveying to the mind the sensation of 

 light ; and this part is the Retina, so named 

 from the thin and delicate membranous network, 

 on which the pulpy extremities of the optic nerves, 

 establishing 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, however, 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 neces- 

 sary that the light, which comes from it, and that 

 light alone, should produce its impression exclu- 

 sively 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 actions would 

 interfere with one another, and only a confused 

 impression would result. The objects in a room, 

 for example, are all throwing light on a sheet of 



VOL. II. D n 



