1835.] Composition of White Light. 173 



Colours, by C. Tomlinson, Esq., published in your two last 

 numbers, which have introduced new methods of obtaining 

 these colours, have very naturally recalled my attention 

 to this part of the subject; and, as Mr. Tomlinson considers 

 the results of his experiments to be opposed to the existing 

 theory, (20.) which it was partly the object of my last paper 

 to support, my observations on them may be given, without 

 any impropriety, as a continuation of these papers ; more 

 particularly, as they will be found to be connected with 

 their principal object. 



If we look at the reflection of objects from the first sur- 

 face of coloured glasses, we find they preserve their proper 

 colours, very little modified by the colour of the glass. This 

 is what might be expected ; for, as the light reflected by the 

 first surface does not enter into the glass, it is not probable 

 that it will be changed by it; the modification, when any is 

 observed, it may be concluded, arises from a mixture of the 

 light reflected by the second surface ; the images formed by 

 the two reflections, under ordinary circumstances, being 

 very little separated, are usually seen blended together. 



In Mr. Tomlinson's first experiments, the circumstances 

 are highly favourable, both to the formation and to the 

 separation of the two images ; for the mercury, under the 

 coloured medium, must insure a total reflection from the 

 second surface ; and the depth of the medium, by consi- 

 derably extending the inclined path of the incident light 

 before it reaches the second surface, must separate the two 

 reflections. 



The latter object is not so completely attained by the 

 method proposed in the second paper, in which a mirror 

 and coloured glasses are substituted for the previous arrange- 

 ment; because, in order to obtain a sufficient reflection 

 from the second surface, the coloured glass must be thin, 

 and the separation of the images is wholly dependent on 

 the distance between the first surface, and the surface which 

 forms the second reflection. 



In repeating Mr. Tomlinson's experiments, which I have 

 done, perhaps not so completely, by putting pieces of coloured 

 glass upon a common looking glass ; I have, in some degree, 

 obviated this objection, by keeping the coloured glass at 

 some distance from the surface of the mirror ; this produces 



