AS PERCEIVED BY THE EYE. 289 



XV.) This method of mixture, besides being adopted by all painters, has been 

 employed by optical writers as a means of obtaining numerical results. The spe- 

 cimens of such mixtures given by D. R. Hay in his works on Colour, and the ex- 

 periments of Professor J. D. Forbes on the same subject, show the importance of 

 the method as a means of classifying colours. There are two objections, however, 

 to this method of exhibiting colours to the eye. When two powders of unequal 

 fineness are mixed, the particles of the finer powder cover over those of the 

 coarser, so as to produce more than their due effect in influencing the resultant 

 tint. For instance, a small quantity of lamp-black, 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 powders 

 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 absorp- 

 tion 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 the separate colours of the sub- 

 stances 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 II. 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 objec- 

 tions to the use of this method are — (1.) The difficulty of obtaining a constant 

 supply of uniformly intense light ; (2.) The uncertainty of the effect of the posi- 

 tion 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 refrangibility, 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. 



