Prof. Challis on the Transmutation of Rays of Light. 525 



prismatic blue and yellow, but takes place just as well with the 

 colours of bodies/^ seeing that at the end of the paragraph marked 

 (9) I endeavour, with whatever success, to account for it theo- 

 retically. 



As little did I question that '^ in making experiments with the 

 spectrum, in order to neutralize, when possible, a prismatic colour 

 of given intensity by another prismatic colour, so as to produce 

 white, two points must be attended to : the place of the second 

 colour in the spectrum must be properly chosen, and the inten- 

 sity of the light properly regulated ; •'■' for I thought I had ex- 

 plained theoretically in the paragraph marked (4), why one of 

 these points must be attended to, and in the paragraph marked 

 (7) why the other must be attended to. There is also another 

 fact which my theory embraces, viz. that when two given colours 

 are mixed, the gradations of colour obtained by varying the pro- 

 portions depend on the source of the light. Thus with sunlight 

 the gradations of a mixture of yellow and blue pass from yellow 

 through diluted yellow, white, and diluted blue, to blue ; whereas 

 in a mixture of yellow and blue from certain sources, the grada- 

 tions pass through green. 



It is necessary that I should now explain in what sense I used 

 the terms *' terrestrial light.^^ The most general composition of 

 a parcel of light may be expressed analytically as follows : — 



i.ii. 27r 27r "^"^^ 



'Um >Y = 7n sin—- (x — at + c) -\- m' sin -^ {a^ — at -\- c') + &c,y5tB;t*T 

 '*o^j; ^ ^ ^ong^ 



t&e number of terms being unlimited, and the values of m^ ni^, 

 &c., and of A,, X', &c., either all different, or some equal. The 

 effect of ordinary reflexion and refraction is to alter all the coeffi- 

 cients m, m', &c. in a given ratio, and sometimes the phases of 

 the rays, the separate rays retaining their individuality, and 

 the character of the light being in no respect altered. The effect 

 of absorption proper is to alter by quantities depending on the 

 absorbing substance the coefficients m, m', &c., reducing some 

 of them to zero while the rays retain their individuality, and 

 to change the phases of the rays, but not to change the vahies 

 of X, X', &c. The effect of true internal dispersion is to alter 

 X, X', &c., and completely to change the character of the light. 

 The last effect may sometimes and in some degree accompany 

 the second. Light modified in the second of these ways I have 

 called terrestrial light, because the modification takes place by 

 the action of the constituent atoms of some terrestrial substance 

 on sunlight. The term may, however, be extended to other light 

 which is not direct sunlight, or sunlight of the first kind. This 

 statement will explain in what sense I considered the coloured 

 light of substances (which I ascribe to absorption) to be " new 

 '..■: ■.■^■\i.\iioo ^fi&^m on '^a p4 w 



