Discovery of his Theory of Colours. 127 



unless Young made the experiments with the rotating coloured 

 disks during the latter part of his course of lectures, he must 

 have made them during the interval between his retirement 

 from the professorship at the Royal Institution and the publi- 

 cation of his lectures on Natural Philosophy in 1807. Young 

 delivered his first lecture before the Royal Institution on Ja- 

 nuary 20, 1802, and was very busy with his lectures until July 

 4, 1803, when he retired. I think that we may fix the date of 

 these remarkable experiments as somewhere between 1803 and 

 1807; and it is highly probable that the theory was never given 

 to the public in a lecture before the Royal Institution, but first 

 appeared in the publication of his Lectures on Natural Philo- 

 sophy. 



That Young should have delayed to bring to the test of ex- 

 periment a plausible hypothesis, when other men would at once 

 have . appealed to the instruments in their laboratories, is ex- 

 plained by the fact that Young "at no period of his life was 

 fond of repeating experiments or even of originating new ones. 

 He considered that, however necessary to the advancement of 

 science, they demanded a great sacrifice of time, and that, 

 when a fact was once established, that time was better employed 

 in considering the purposes to which it might be applied or the 

 principles which it might tend to elucidate." Indeed this 

 peculiarity receives abundant confirmation from his own words. 

 Thus, in the Bakerian Lecture, already so often referred to, he 

 says : — "Nor is it absolutely necessary in this instance [in 

 speaking of the proofs to be adduced in support of the undula- 

 tory theory of light] to produce a single new experiment ; for 

 of experiments there is already an ample store." And in a let- 

 ter written in November 1827, to his sister-in-law, Mrs. Earle, 

 on the respective honours given by Herschel, in his ' Optics,' 

 to Young and Fresnel, he says, " And acute suggestion was 

 then, and indeed always, more in the line of my ambition than 

 experimental illustration." Young carried his opinion of the 

 secondary importance of experiments so far as even to object 

 to the increase of the fund left by Wollaston to the Royal 

 Society to aid experimental inquiries, in these words : — " For 

 my part, it is my pride and pleasure, as far as I am able, to 

 supersede the necessity of experiments, and more especially of 

 expensive ones." 



