PREFACE. XXV 



a violet. This theory had been revived by Helmholtz who endeavoured to find for it a 

 physiological basis. Maxwell however devoted himself chiefly to the invention of accurate 

 methods for combining and recording mixtures of colours. His first method of obtaining 

 mixtures, that of the Colour Top, is an adaptation of one formerly employed, but in 

 Maxwell's hands it became an instrument capable of giving precise numerical results by 

 means which he added of varying and measuring the amounts of colour which were 

 blended in the eye. In the representation of colours diagrammatically he followed Young 

 in employing an equilateral triangle at the angles of which the fundamental colours were 

 placed. All colours, white included which may be obtained by mixing the fundamental 

 colours in any proportions will then be represented by points lying within the triangle. 

 Points without the triangle represent colours which must be mixed with one of the funda- 

 mental tints to produce a mixture of the other two, or with which two of them must be 

 mixed to produce the third. 



In his later papers, notably in that printed in the Philosophical Transactions, he 

 adopts the method of the Colour Box, by which different parts 6f the spectrum may be 

 mixed in different proportions and matched with white, the intensity of which has been 

 suitably diminished. In this way a series of colour equations are obtained which can be 

 used to evaluate any colour in terms of the three fundamental colours. These observations 

 on which Maxwell expended great care and labour, constitute by far the most important 

 data regarding the combinations of colour sensations which have been yet obtained, and 

 are of permanent value whatever theory may ultimately be adopted of the physiology of the 

 perception of colour. 



In connection with these researches into the sensations of the normal eye, may be 

 mentioned the subject of colour-blindness, which also engaged Maxwell's attention, and is 

 discussed at considerable length in several of his papers. 



Geometrical Optics was another subject in which Maxwell took much interest. At an early 

 period of his career he commenced a treatise on Optics, which however was never completed. 

 His first paper "On the general laws of optical instruments," appeared in 1858, but a brief 

 account of the first part of it had been previously communicated to the Cambridge Philosophical 

 Society. He therein lays down the conditions which a perfect optical instrument must fulfil, 

 and shews that if an instrument produce perfect images of an object, i.e. images free from 

 astigmatism, curvature and distortion, for two different positions of the object, it will give 

 perfect images at all distances. On this result as a basis, he finds the relations between 

 the foci of the incident and emergent pencils, the magnifying power and other characteristic 

 quantities. The subject of refraction through optical combinations was afterwards treated 

 by him in a different manner, in three papers communicated to the London Mathematical 

 Society. In the first (1873), "On the focal lines of a refracted pencil," he applies Hamilton's 

 characteristic function to determine the focal lines of a thin pencil refracted from one 

 isotropic medium into another at any surface of separation. In the second (1874), "On 

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