XXXV.] COLOURS AND THEIR COMBINATION. 113 



or any other colour may be formed by combining them in 



suitable proportions. ^ 



We thus find that, as a physical fact, rays of all colours are 



equally primary. But Professor Clark Maxwell ^ has found 



reason, from observations on colour-blind persons, to think that, Ijut there 



as a physiological fact, sensations of all colours are not equally ™^7 ^'^'" 



^ T'hysio- 

 primary; but that all other sensations of colour are compounded logical 



of the three primaries, red, green, and blue. sense. 



I shall conclude this note with some remarks on the mathe- Further 

 matical theory of chromatic octaves and complementaries. ^a*"^- 

 Logically they ought to have come earlier, but I think it better considera- 

 to leave the statement of the elementary laws of colour unen- *'°°^" 

 cumbered by even the simplest mathematics. 



In what follows let us, as in musical science, speak not of the 

 wave-lengths, but of the frequency of the waves ; that is to say, Wave fre- 

 the number of waves in a given time. The frequency is ofV^eiicy. 

 course inversely as the length. 



Having ascertained the wave-frequency to which either a 

 sound or a colour is due, we find the wave- frequency of its 

 octave by multiplying by two, or of its lower octave by dividing 

 by the same. Extreme purple is the octave of extreme red, A colour 

 and on tlie chromatic circle drawn above these are separated ^^^ ^^^ 

 by an arc of 360°. Now, the pairs of complementaries, as I 360° apart 

 have laid them down, are separated, each colour from its °." V*'' 

 complementary, by an arc of 180°. If this circular arrange- 

 ment of colours represents any truth of nature, it might conse- 

 quently be supposed that, having ascertained the wave-frequency 

 of any colour, we should be able to find that of its complemen- 

 tary by multiplying or dividing, as the case may be, by the 

 square root of two. That is to say, if we arrange the colours, 

 from red to its octave where purple turns red again, round the 

 360 degrees of a circle, in such a way that any two colours sepa- 

 rated by equal arcs shall have their wave-frequencies in. equal 

 ratios ; then, as the wave-frequencies of the two reds which are 



^ See Professor Clark Maxwell's paper, already referred to. 



2 See the same paper. According to Professor Clark Maxwell, most 

 colour-blind persons (so called) are insensible to any difference between 

 red, yellow, and green, but sensible to the difference between these and 

 blue or violet. But there is another less common kind of colour- 

 blindness, which consists in insensibility to the difference between green 

 and bine. 



VOL. II. [ 



