422 Prof. Stokes's Remarks on Prof. Challis's paper, 



to the extent of some researches of my own. I have, however, 

 on some points expressed opinions, respecting the justice of which 

 it is only one who is famihar with certain classes of optical expe- 

 riments who can feel the confidence that I entertain. 



From the paragraph commencing at the foot of page 330, it 

 is plain that Professor Challis has made some confusion between 

 three perfectly distinct things : Sir David Brewster's controverted 

 analysis of the solar spectrum by means of absorbing media* ; 

 his discovery of the phsenomenon of internal dispersion f; and my 

 own discovery, that a beam of rays of prismatic purity (whether 

 belonging to the visible or invisible portion of the spectrum is 

 indifferent) may, by their action on certain media, produce light 

 which may be decomposed by the prism into portions extending 

 over a wide range of refrangibility, and having colours answering 

 to their refrangibilitiesj. 



As to the first, it was asserted by Sir David Brewster that 

 light of prismatic purity may have its colour changed by passing 

 through absorbing media. This has nothing to do with "internal" 

 or "epipolic'' dispersion, or "fluorescence.'' Glass coloured 

 blue by cobalt, for instance, has none of these properties, although 

 it is one of the media which exhibit most strikingly the phae- 

 nomena adduced by Sir David Brewster, Were such a change 

 of colour made out, it would be a point of the utmost importance 

 to consider in reference to any physical theory of light. But 

 while none deny that the appearances are as stated by Sir David 

 Brewster, the inference to be drawn from those appearances 

 remains open to discussion. Airy §, Helmholtz ||, and Bernard^, 

 by operating in a different manner, have come to the conclusion 

 that the colour is not changed ; and Helmholtz has attributed 

 the apparent change partly to the mixture of a very small quan- 

 tity of stray light, partly to the effects of contrast. Having 

 been much in the habit of analysing the light transmitted by 

 coloured solutions, and having repeatedly seen the phsenomena 

 on which Sir David Brewster relies, I may be permitted to ex- 

 press my belief that the change of colour is only apparent, being 

 an illusion depending upon contrast, and that this is one of the 

 cases in which the direct evidence of the senses must be con- 

 trolled. Were the change of colour real. Prof. Challis's state- 

 ment (p. 330), that " experiment has proved that both the colour 

 and the angle of refraction for a given angle of incidence depend, 



* Edinburgh Transactions, vol. xii. p. 123. 



t Edinb. Trans, vol. xvi. p. 1 1 1 ; and Phil. Mag. vol. xxxii. (1848) p. 401 . 

 X Philosophical Transactions for 1862, p. 463. 

 § Phil. Mag. vol. xxx. p. 73. 

 II Poggendorff's Annalen, vol. Ixxxvi. p. 601. 



5r Report of the Meeting of the British Association at Liverpool in 1864, 

 2nd part, p. 6. 



