126 Prof. A. M. Mayor on the History of Young's 



statement as to the composition of yellow light; and it is 

 therefore probable that Wollaston communicated orally this 

 view of the subject to Young. Every student of optics now 

 knows that the description already given of Fraunhofer's ob- 

 servation on the colour composition of the spectrum is the cor- 

 rect one ; yet the errors of observation of Wollaston and of 

 Young were errors which led to a great discovery, as we shall 

 see on the further examination of the history of this beautiful 

 and comprehensive theory of colour. 



Further on in the % Natural Philosophy ' we read that " the 

 sensations of various kinds of light may also be combined in a 

 still more satisfactory manner by painting the surface of a 

 circle with different colours, in any way that may be desired, 

 and causing it to revolve with such rapidity that the whole 

 may assume the appearance of a single tint, or of a combina- 

 tion of tints, resulting from the mixture of the colours." These 

 experiments wore evidently first made by Young, and are fully 

 described in the text and perfectly illustrated in the coloured 

 disks in the plates of Young's work. These experiments have 

 been carefully repeated by Helmholtz, Maxwell, and others ; 

 and of their general accuracy there is no doubt. We can rea- 

 dily imagine the delight with which Young must have viewed 

 these beautiful experiments, which, however, together with 

 other truths unfolded by him, were destined to remain unnoticed 

 " until a later generation, by slow degrees, arrived at the dis- 

 covery of his discovery." 



It must now recur to the reader to inquire ivhen were made 

 these experiments which first confirmed Young's hypothesis 

 and placed it among the bestrestablished truths of optical 

 science, and why it was that Young should for so long a time 

 have been satisfied with a hypothetical statement of his views 

 on the colour-sensations, and should have deferred to bring 

 those views to the test of experiment. For reasons already 

 stated, Young, in July 1802, changed his three elementary 

 colour-sensations, red, yellow, and blue, to red, green, and vio- 

 let. The experiments with the rotating coloured disks were 

 first published in 1807. Young printed the syllabus of his 

 first course of lectures on January 19, 1802, in a volume of 

 250 pages. I have not been able to procure a copy of this 

 syllabus ; but evidently it does not contain even the corrected 

 statement of his theory of colour ; for that was based on Wol- 

 laston' s observations, which appeared subsequently to the 

 syllabus, on June 24, 1802*. It is therefore evident that, 



* Professor Tyndall has recently informed me that Young's syllabus 

 gives red, yellow, and blue as the three elementary colour-sensations. 



