84 



I consider to be disposed of by the suppression of the adjacent 

 colours, as recorded above. 2ndly, by extinction of a yellow element 

 of colour over the space DE, allowing a substratum of green to sur- 

 vive ; or, which comes to the same, by the extinction of the red ele- 

 ment over the same space, which, by its combination with (an as- 

 sumed elementary) green, produced the original brilliant straw-yellow. 

 And 3rdly, by admitting as a principle, that our judgment of colours 

 absolutely, in se, and independent of contrast, is influenced by the 

 intensity of the light by which they affect the eye, and that very 

 vivid illumination enfeebles or even destroys the perception of 

 colour. As the apparent change of colour from pale-yellow to green 

 in the cases above related was always accompanied with a great 

 diminution of general intensity, it occurred to me to produce such 

 diminution by optical means, which should operate equally on all the 

 coloured rays, and diminish all their intensities in the same ratio. 

 This was accomplished by viewing the spectrum (as projected on 

 purely white paper) by reflexion on black glass, or by two successive 

 reflexions in different planes, and I found the very same effect to 

 take place. That portion DE of the spectrum which in the unre- 

 flected state appeared dazzlingly bright and nearly colourless, was 

 seen by one such reflexion, and still more so by two, green. The 

 extension of the green region was greater, and the limitation of the 

 yellow portion more complete, according to the amount of illumina- 

 tion destroyed by varying the angles of incidence on the glasses. 

 When much enfeebled by two cross reflexions, the aspect of the 

 spectrum was that represented in Chevreul's coloured picture of it 

 from the line A to H. When enfeebled by other means, as by view- 

 ing the spectrum thrown on a blackened surface, the effect was 

 exactly the same. 



The last of our three alternatives, then, would appear to be esta- 

 blished as the true explanation ; and in respect of the second, it is 

 eliminated by the consideration that neither the slight degree of 

 coloration in the bluish papers, or the tint of the pale-yellow ones 

 which effected the change, would give rise to so great a preferential 

 extinction of yellow or red rays as an explanation founded on that 

 alternative would require. The phenomenon is certainly a very 

 striking one, and has created great surprise in those to whom I have 

 shown it. 



