468 JAMES CLERK MAXWELL. 



three heavy particles placed at the angular points of the 

 triangle, the weight of each particle being proportional to the 

 quantity of the corresponding light employed in the mixture. 



Although the light of the spectrum which corresponds 

 to any particular spectral colour is simple in its constitution, 

 consisting of a definite number of waves per second, and 

 ordinary white light is a combination of an infinite number 

 of such rays, varying in rapidity of vibration from the 

 extreme red to the extreme violet, yet the mechanism of 

 vision appears to be of a threefold character, so organised 

 that light of each particular wave-length affects in different 

 proportions the three colour senses, the hue depending on 

 the relative amounts to which they are severally excited, 

 as explained by Young. An analogy may help to make 

 this distinction clearer ; thus, it has been customary to speak 

 of the heating power, the illuminating power, and the photo- 

 graphic power, of any particular kind of light ; and in the 

 solar spectrum formed by a glass prism the greatest heating 

 power is possessed by the ultra red rays, the greatest 

 illuminating power ( judged by normal eyes) by the greenish 

 yellow rays, and the greatest photographic power (in relation 

 to silver salts) by the ultra-violet rays ; but we do not 

 thereby imply that there are three separate classes of rays 

 in solar light, consisting respectively of heating rays, illumi- 

 nating rays, and chemical rays : we simply mean that any 

 particular homogeneous ray possesses each of these three 

 powers to a certain extent. Similarly any ray of the spec- 

 trum may be capable of stimulating each of three independ- 

 ent colour sensations, but it does not follow that it consists 

 physically of three different kinds of light. 



In the Philosophical Magazine for 1852 there is an 

 account of an experiment by Helmholtz, which consisted in 

 mixing the colours of the spectrum by forming two spectra 

 from slits at right angles to each other. It is obvious that 

 if the breadth of each spectrum be equal to its length, every 

 pair of colours in the whole spectrum will in this way be 

 superposed. In Helmholtz's experiment it appeared that a 

 mixture of yellow and indigo produced white. 



