266 A. M. Mayer— History of Young’s Discovery, ete. 
must now recur to the reader to inquire, when were made 
these experiments which first confirmed Young’s hypothesis and 
placed it among the best established 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 
color-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 color-sensations, 
red, yellow and blue, to red, green and violet. e experi- 
ments with the rotating colored discs were first published in 
1807. Young printed the syllabus of his first course o 
tures on January 19th, 1802, in a volume of 250 pages. I 
have not been able to procure a copy of this syllabus, but evi- 
dently it does not contain even the corrected statement of his 
theory of color, for that was based on Wollaston’s observation, 
which appeared subsequently to the syllabus, on June 24th, 
1802. lt is therefore evident that unless Young made the ex- 
periments with the rotating colored discs during the latter part 
of his course of lectures, he must have made them during the 
that we may fix the date of these remarkable experiments as 
somewhere between 1808 and 1807, and it is highly probable 
that the theory was never given to the public in a lecture be- 
fore the Royal Institution, but first appeared in the publication 
of his Lectures on Natural Philosophy. 
That Young should have delayed to bring to the test of ex- 
paige a plausible hypothesis, when other men would at once 
ave appealed to the instruments in their laboratories, 1s exX- 
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 scl- 
ence, they demanded a great sacrifice of time; and that, when 
a fact was once established, that time was better employed in 
principles which it might tend to elucidate.” Indeed, this 
peculiarity receives abundant confirmation from his own words ; 
experiments there is already an ample store ;” and ina letter 
written in November, 1827, to his sister-in-law, Mrs. Earle, on 
the respective honors given by Herschel, in his Optics, to Young 
