Prof. Challis on the Unduhtory Theory of Light. 337 



of yellow and blue colours made by chalk pencils, I constantly 

 found that even when the eye was near enough to distinguish 

 the spaces easily, the whole appeared to be suffused by a tinge 

 of green. Now, although this diffusion in both kinds of ex- 

 periment may be referable to the manner in which the organ of 

 sight is acted upon by the rays, it proves not the less that a 

 combination of yellow and blue has the same effect in producing 

 green, whether the light come directly from the sun, or is what 

 I called terrestrial light. For this reason I withdraw the dis- 

 tinction I endeavoured to establish between solar light and ter- 

 restrial light. 



With reference to the same subject, I take this opportunity 

 to state that I have made experiments for showing the effects 

 of combining colours by means of revolving disks, the disks 

 being divided into spaces covered alternately with the two 

 colours to be compounded. The apparatus I used was pro- 

 fessedly made according to directions contained in Professor 

 Maxwell's paper on this subject, and among the different sets 

 of colours was one which was intended to show that yellow and 

 blue combined do not produce green. The result in this in- 

 stance was certainly a dirty white ; but according to my sight 

 the blue and the yellow had scarcely any resemblance to pris- 

 matic blue and yellow. On substituting for them the very same 

 chalk colours that I used in the above-mentioned experiment of 

 parallel spaces, I found that the result was decidedly green. 

 It may be that the colours I used were not pure colours; 

 but the fact that one appeared blue and the other yellow was 

 owing to the predominance of blue or yellow solar rays, and 

 the predominant tint of the compound was determined ac- 

 cordingly. 



For these reasons, drawn, it will be seen, in part from per- 

 sonal observations, I hold that sunlight and terrestrial light are 

 not essentially different, and that, in accordance with the ma- 

 thematical theory of the composition of colours given in the 

 above-cited article, combinations of yellow and blue, with either 

 kind of light, have the effect of producing green. 



(4) It having been suggested to me to employ Angstrom's 

 values of X, given in PoggendorfFs Annalen for November 

 1864, for testing my Theory of the Dispersion of Light con- 

 tained in the Supplementary Number of the Philosophical 

 Magazine for December 1864, I have calculated as follows for 

 this purpose. It was considered sufficiently accurate to obtain 

 the values of X for the rays C, D, F, and Gr from those for the 

 rays B, E, and H, and the given values of /uu by mere interpola- 

 tion, and to regard the differences of the results deduced from 

 the old and the new values of \ as the same that would have 



