1822.] of Vision with Regard to Colours, ig|J 



set ; yet it may continue sensible, or it may be so in an increased 

 degree to the other sets of rays. 



When the retina has been for some time powerfully acted 

 upon by rays of light which have produced the sensation seeing 

 red, that sensation will at length cease, although the action of 

 those rays be continued ; yet if at such time the action of those 

 rays be even continued, or if rays of mixed light be admitted to 

 the retina, or if all rays of light be excluded from the retina, the 

 .sensation seeing green will arise. In like manner, the long conti- 

 nuance oi seeing green leads to the production of seeing red. 



When after the long continuance of the sensation seeing red, 

 the senssition seeing green arises in the manner just described 

 (and vice versa), although the sensation thus undergoes a change 

 with regard to what we term colour, it continues precisely the 

 same with regard to what we term shape, and extent of surface; 

 so that a retina which has become insensible to the action of 

 one of the prismatic rays may still be adequate to all the com- 

 mon purposes of vision, since, although there may be a defect of 

 .sensation with regard to colour, it does not follow that there is 

 any defect with regard to shape and extent of surface. 



It appears then that there is a certain state of the retina 

 necessary, in order that the mixed rays of hght so affect it as to 

 produce vision. It also appears, that there is a particular state 

 of the retina requisite, in order that each separate and distinct 

 set of the rays of light so affect it as to produce a corresponding 

 pecuHar and distinct kind of vision. There is, for instance, a 

 state of the retina in which all the rays of light, whether blended 

 or distinct, so affect the retina as to produce vision. Such a 

 state may be termed general sensibility of the retina. There is 

 a state of the retina in which all the rays of light, whether 

 blended or distinct, fail to affect the retina in such a manner as 

 to produce vision. Such a state may be termed general insensi- 

 bility of the retina. There is a state of the retina in which a 

 certain set of the prismatic rays (as, for instance, the red rays) 

 fail to produce a corresponding kind of vision. There is also a 

 state of the retina in which only one set of the prismatic rays (as^ 

 for instance, the red rays) affects the retina in such a manner as 

 to produce vision. In such cases, there \^ partial sensibility and 

 Also partial insensibility of the retina. 



That state of retina then which fits it for being so acted upon 

 by one particular set of the prismatic rays as to produce vision, 

 does not necessarily fit it for being so acted upon by another set, 

 or by all the other sets, of the prismatic rays. 



It appears then that whenever light acts upon the retina in 

 ^uch manner as to produce vision, it produces a certain condi- 

 tion or state of the retina which is essential to the existence of 

 that kind of sensation which is termed seeing. It appears also, 

 that in order that each distinct set of the prismatic rays so affect 

 the retina as to produce a distinct corresponding kind of tha 



Neiv Series, vol. iii. k 



