viz EETINAL EXCITATION 367 



reduced them to a stricter and more accurate formula ; to > 

 Helmholtz we owe the complete, systematic study and experi- 

 mental demonstration of these laws. 



Just as the decomposition of white light yields all the hues 

 and colours perceptible in the spectrum, so by the mixture or 

 superposition of certain spectral colours in varying proportions 

 we can artificially obtain all the more or less complex hues 

 of nature. 



The physical method is the most perfect means of obtaining 

 colour-mixtures, and consists in allowing rays of different wave- 

 length, previously separated by two prisms, to act simultane- 

 ously upon the retina. The application of this method involves 

 a complicated apparatus which is described in all Text -books 

 of Physics. 



The physiological method is simpler, and consists in letting 

 the colours to be mingled fall on the eye in succession instead of 

 simultaneously, at such a rate that the persistence of the images 

 causes superposition or mixture of the colours on the retina. 

 Pigments can be used for this purpose instead of spectral colours, 

 for though less saturated than the latter they contain the different 

 tones of colour. . 



Various contrivances have been employed to obtain a physio-\ 

 logical mixture of pigment colours. The most ingenious and 

 that usually adopted is known as Maxwell's colour -discs. As 

 shown by Fig. 175, these are circular papers of different colours 

 as opaque and as saturated as possible with a radial slit, by 

 which they can be superposed so as to present two or three 

 differently coloured sectors, the area of which can be varied at 

 will by the experimenter. These are placed on a metal disc, 

 fixed in the centre by a screw, and are then rotated by clockwork ; 

 the visual sensation thus produced varies according to the colours 

 employed, their saturation, and the relative proportion of the 

 sectors upon the disc. 



The mixture of two or more spectral colours produces a new 

 colour, or rather a new visual sensation, which does not merely 

 result from the superposition of the two component chromatic 

 sensations, because the new colour is always less saturated than 

 its two components. The luminous intensity is not diminished 

 in the mixture, but part of the chromatic quality disappears, and 

 is replaced by white. 



When two colours of the spectrum known as complementary 

 are mixed, all chromatic quality in the sensation disappears, and 

 white or grey light only is perceived. This is the fundamental 

 law of colour -mixtures, and it claims our attention in the 

 first place. 



Helmholtz demonstrated that not merely can white or grey 

 light, i.e. colourless sensation, be obtained by mixing one or more 



