1822.] of Vision with Regard to Colours. 131 



The colour then which is seen in any case is connected with 

 the presence of rays of hght only in the relation of effect and 

 cause. 



Lio-ht is divisible by the prism into several sets of rays, whereof 

 one set excites the sensation seeing red, the next seeing orange, 

 the third seeing yellow, the fourth seeing green, the fifth seeing 

 blue, and the sixth seeing violet. These several sets are distin- 

 guished by the names red, orange, yellow, green, blue, and 

 violet rays. The red, yellow, and blue rays are termed primary 

 rays. The orange ray is considered as a compound of the red 

 and yellow rays ; the green, as a compound of the yellow and 

 blue rays ; and the violet, as a compound of the blue and red 

 rays. The oran2i:e, the green, and the violet rays are, therefore, 

 termed compound rays. 



Let us suppose a retina to be insensible to red rays. In such 

 a case, it would be sensible to the two other primary rays, 

 namely, to the yellow and to the blue rays. It would be sensi- 

 ble also to the ray compounded of these two rays ; namely, to 

 the green ray. The action of light then upon such a retina 

 might produce the sensations seeing yellow, seeing green, and 

 seeing blue. It would not produce the sensation, seeing red. 

 And as the orange and the violet rays are each in part com- 

 pounded of the red ray, the sensations seeing orange, or seeing 

 violet, would not be produced by the action of light upon such a 

 retina. 



Let us suppose a retina to be insensible to yellow rays. In 

 such a case it would be sensible to the two other primary rays ; 

 namely, to the red, and to the blue rays, and also to the ray 

 compounded of these two rays ; namely, to the violet ray. The 

 action of light then upon such a retina might produce the sensations 

 seeifig red, seeing blue, and seeing violet ; but it would not produce 

 seeing yellow. And as the orange and the green rays are each 

 in part compounded of the yellow ray, the sensations seeing 

 orange and seeing green would not be produced by the action of 

 light upon such a retina. 



Let us suppose a retina to be insensible to blue rays. In such 

 a case, it would be sensible to the two other primary rays ; 

 namely, to the red and to the yellow rays, and also to the ray 

 compounded of these two ; namely, to the orange ray. The 

 action of light then upon such a retina might produce the sensa- 

 tions seeing red, seeing orange, and seeing yellow; but it would 

 not produce seeing blue. And as the green and the violet rays 

 are each in part compounded of the blue ray, the sensations 

 seeing green and seeing violet, would not be produced by the 

 action of hght upon such a retina. 



If a retina be insensible to the orange ray, it may be so from 

 being insensible either to the red, or to the yellow rays, of 

 which two rays the orange ray is compounded. In the former 

 case, seeing blue, seeing yellow, and seeing green, may arise : in 



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