198 JAMES CLERK MAXWELL. [CHAP. VII. 



his own accord in teaching, undertook the task of 

 examining Cheltenham College, and submitted to the 

 routine which belonged to his position at Cambridge. 

 As a foretaste of delights in store, he had spent 

 the evenings of the Senate-house days in (physico-) 

 magnetic seances with his friends. But when actually 

 emancipated he seems to have reverted principally 

 at first to his beloved Optics. He makes inquiries 

 about a microscope manufactory at Zurich ; reads 

 Berkeley's Theory of Vision taking up Mill's Logic 

 by the way, and finding there by no means the 

 last word on the relation of sense to knowledge ; 

 looks up his stock of coloured papers furnished 

 by D. K. Hay, and sets to work spinning and 

 weaving the different rays ; inquires him out colour- 

 blind persons on all sides ; and invents an instru- 

 ment for inspecting the living retina, especially 

 of dogs. By and by it is the Art of Squinting which 

 again has charms for him, and he combines it with 

 the teaching of solid and spherical geometry, by 

 drawing wonderful stereoscopic diagrams. So far, his 

 investigations oscillate between colour and form. 

 But even the fascination of the Colour-Top * cannot 



1 The "colour-box," though perfected only in 1862, was in full 

 operation in the study at Glenlair several years before this I think as 

 early as 1850. And even then he had begun spinning coloured discs, 

 proportionately arranged, so as to ascertain the true " mixture of 

 colours." He was fond of insisting, to his female cousins, aunts, etc., 

 on the truth that blue and yellow do not make green. I remember 

 his explaining to me the difference between pigments and colours, and 

 showing me, through the " colour-box," that the central band in the 

 spectrum was different from any of the hitherto so-called " primary 



