83 



in question a dazzling very pale straw-yellow, hardly distinguishable 

 from white, yet as the photographic action proceeded, and the 

 translucency of the paper began to be somewhat diminished also by 

 incipient drying, very nearly the whole of that space became occu- 

 pied by a full and undeniable green colour, so as to give the idea of 

 a distinctly four-coloured spectrum red, green, blue, and violet; 

 the yellow being in some instances almost undiscernible, and in 

 others limited to a mere narrow transitional interval rather orange 

 than yellow. It was at the same time evident that a great extinction 

 of light (illumination independent of colour) had also been operated, 

 the vivid glare of the part of the spectrum in question being reduced 

 to a degree of illumination considerably inferior to the red part, or, 

 at all events, not much superior. The change of colour was far 

 greater than could be attributed to any effect of contrast, and was 

 proved decisively not to be due to that cause by hiding the adjacent 

 red and blue when the green remained unaffected in apparent tint. 



When, for the photographic preparations wetted as described, 

 ordinary, dry, coloured papers were substituted, the change of colour 

 in question was always produced whenever the thickness of the paper 

 and its absorptive power were not such as to destroy or very much 

 enfeeble the more refrangible light. Taking, as a term of compa- 

 rison, a purely white, wove, writing-paper, I found that the substi- 

 tution of writing-paper, tinted with the ordinary cobalt blue com- 

 monly met with, sufficed to give a very great extension of the green, 

 almost to the extinction of the yellow, while, when the papers used 

 were pale-yellow or clay-coloured, answering to the tints called 

 "buff" or "maize" (nearly approximating to Chevreul's orange 

 4 and 3), and which might naturally have been expected to transmit 

 yellow rays more abundantly at all events than the blue, the spectra 

 (viewed at the back of the papers) were particularly full and abun- 

 dant in green, occupying the whole of the debateable ground. In 

 the case of the former, a narrow yellow space was seen, and the blue 

 was very much enfeebled, and separated from the green by a very 

 perceptible suddenness of transition. With the latter the green was 

 finely exhibited, and the yellow confined to a narrow orange-yellow 

 border : the blue and violet much enfeebled. 



On further considering these facts, there seemed to be but three 

 ways of accounting for them : 1st, by the effect of contrast. This 



