VOL. XI.] PHILOSOPHICAL TRANSACTIONS. 337 



But this I was not surprised at, having observed, on dissecting the eye, that 

 objects are painted on the retina after a contrary posture to what they appear to 

 sight. Having thus rendered the coloured image much more tractable than for- 

 merly it was, I conceived good hopes of some further discovery in this point. 



In pursuance then of my former suspicion, having fixed my pri^sm in a steady 

 \ postiire, I caused the paper C. (fig. 2, 3, pi. xi.) to be applied close to the paper 

 circle a b d : whereupon the former violet d, and scarlet colour of C, vanished 

 into whiteness. Next, I removed the circle from the shutter, and placed it in 

 the open window, supported only by the edge d : on which, to my astonish- 

 ment, all the former colours exchanged postures in the retina, the scarlet now 

 appearing below, the violet above ; the intermediate colours scarcely discernible. 

 And here, by the bye, it is very remarkable, that during this observation I clearly 

 perceived both blue and scarlet light to be transparent, being able to discern 

 several objects through both, namely, steeples opposite to my window. Whence 

 it follows, that these colours do in great part arise from the neighbouring light. 

 Lastly, I placed the paper circle anew, so as the one half b, was fastened to the 

 shutter, the other semicircle a, being exposed to the open air. On which the 

 semicircle a, became bordered with violet above, and scarlet below: but the 

 other semicircle b, quite the reverse. Hence I make the following inferences. 



First, that not only the light reflected from the paper circle, but also from the 

 ambient air, has great influence on the coloured image, especially as to the 

 violet and scarlet colours. Whence perhaps it will not hereafter seem strange, 

 that the coloured spectrum on the wall is so long, but only that the breadth is 

 not greater. Secondly, were there a more luminous body behind the sun, we 

 should in all likelihood have the colours of the spectrum in a contrary situation 

 to what they appear in at present : whence, thirdly, it seems to follow, that the 

 present situation and order of colours arise not from any intrinsical property of 

 refrangibility, as maintained by Mr. Newton, but from contingent and extrinsi- 

 cal circumstances of neighbouring objects. For accordingly as the body be- 

 hind the paper-circle was more or less illuminated than the circle itself, all the 

 several colours changed their situation. 



8. The next experiment was made in order to Mr. Newton's doctrine of pri- 

 > mary colours, as prop. 5. Having covered the hole in the window-shutter with 

 a thin slice of ivory, the transmitted light appeared yellow; but on adding three, 

 four, and more slices, it became red. Whence it seems to follow, that yellow- 

 ness of light is not a primary colour, but a compound of red, &c. 



Q. The last experiment was made in reference to Mr. Newton's 12th prop, 

 where from his own principles he renders a very plausible reason of a surprising 

 phenomenon, related by Mr. Hook, namely of two liquors, the one blue, the 



VOL. II. X X 



