EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 143 



mixed with a large quantity of chalk, will produce a mixture which is nearly 

 black. Although the powders generally used are not so different in this respect 

 as lamp-black and chalk, the results of mixing given weights of any coloured 

 powders must be greatly modified by the mode in which these powders have 

 been prepared. 



Again, the light which reaches the eye from the surface of the mixed pow- 

 ders consists partly of light which has fallen on one of the substances mixed 

 without being modified by the other, and partly of light which, by repeated 

 reflection or transmission, has been acted on by both substances. The colour of 

 these rays will not be a mixture of those of the substances, but will be the 

 result of the absorption due to both substances successively. Thus, a mixture of 

 yellow and blue produces a neutral tint tending towards red, but the remainder 

 of white light, after passing through both, is green ; and this green is generally 

 sufficiently powerful to overpower the reddish gray due to ^he separate colours 

 of the substances mixed. This curious result has been ably investigated by 

 Professor Helmholtz of Konigsberg, in his Memoir on the Theory of Compound 

 Colours, a translation of which may be found in the Annals of Philosophy for 

 1852, Part 2. 



(2) Mixture of differently-coloured Beams of Light by Superposition 



on an Opaque Screen. 



When we can obtain light of sufficient intensity, this method produces the 

 most beautiful results. The best series of experiments of this kind are to be 

 found in Newton's Opticks, Book I. Part n. The different arrangements for 

 mixing the rays of the spectrum on a screen, as described by Newton, form 

 a very complete system of combinations of lenses and prisms, by which almost 

 every possible modification of coloured light may be produced. The principal 

 objections to the use of thia method are (1) The difficulty of obtaining a con- 

 stant supply of uniformly intense light ; (2) The uncertainty of the effect of 

 the position of the screen with respect to the incident beams and the eye of 

 the observer ; (3) The possible change in the colour of the incident light due 

 to the fluorescence of the substance of the screen. Professor Stokes has found 

 that many substances, when illuminated by homogeneous light of one refrangi- 

 bility, become themselves luminous, so as to emit light of lower refrangibility. 

 This phenomenon must be carefully attended to when screens are used to exhibit 

 light. 



