VISION. 317 



globe, but it has introduced us to the knowledge of the bo- 

 dies which compose the solar system, and of the countless 

 hosts of stars which are scattered through the firmament, 

 thus expanding our views to the remotest confines of crea- 

 tion. As the perceptions supplied by this sense are at once 

 the quickest, the most extensive, and the most varied, so 

 they become the fittest vehicles for the introduction of other 

 ideas. Visual impressions are those which in infancy, fur- 

 nish the principal means of developing the powers of the un- 

 derstanding: it is to this class of perceptions that the philoso- 

 pher resorts for the most apt and perspicuous illustrations of 

 his reasonings; and it is also from the same inexhaustible 

 fountain that the poet draws his most pleasing and graceful, 

 as well as his sublimest imagery. 



The sense of Vision is intended to convey to its posses- 

 sor a knowledge of the presence, situation, and colour of 

 external and distant objects, by means of the light which 

 those objects are continually sending off, either spontane- 

 ously, or by reflection from other bodies. It would appear 

 that there is only one part of the nervous system so pecu- 

 liarly organized as to be capable of being affected by lumi- 

 nous rays, and conveying 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 extre- 

 mities of the optic nerves^ establishing an immediate com- 

 munication 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 per- 

 ception 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 necessary that the light, which comes from 

 it, and that light alone, produce its impression exclusively 



