Discovery of Ms Theory of Colours. 119 



hypothesis, were entirely hypothetical, not having been based 

 on any observation or experiment either of his own or of others, 



The next publication by Young on his theory of colour takes 

 place in the following year, and is contained in the following 

 short paragraph, incidentally written toward the conclusion of 

 a paper read by him before the Royal Society on July 1, 1802, 

 and entitled " An account of some cases of the production of 

 colours, not hitherto described." 



" In consequence of Dr. Wollaston's correction of the de- 

 scription of the prismatic spectrum, compared with these obser- 

 vations, it becomes necessary to modify the supposition that I 

 advanced in the last Bakerian Lecture, respecting the propor- 

 tions of the sympathetic fibres of the retina ; substituting red, 

 green, and violet for red, yellow, and blue, and the numbers 7, 

 6, and 5 for 8, 7, and 6." 



It thus appears that Young changed his three elementary 

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

 violet, " in consequence of Dr. Wollaston's correction of the 

 description of the prismatic spectrum." In order to under- 

 stand fully the ground for this modification of his hypothesis, 

 one will be obliged to read the following abstract from Dr. 

 Wollaston's paper, published in the ' Transactions of the Royal 

 Society ' for 1802 ; and we are pleased to detain the reader 

 with this paper, because it contains the first publication of the 

 observation of those dark lines to which modern spectroscopic 

 research has given such important and prominent significance. 



" I cannot conclude these observations on dispersion -without 

 remarking that the colours into which a beam of white light 

 is separable by refraction, appear to me to be neither 7, as they 

 usually are seen in the rainbow, nor reducible by any means 

 (that I can find) to 3, as some persons have conceived ; but 

 that, by employing a very narrow pencil of light, four primary 

 divisions of the prismatic spectrum may be seen, with a degree 

 of distinctness that, I believe, has not been described nor ob- 

 served before. 



" If a beam of daylight be admitted into a dark room by a 

 crevice Jq of an inch broad, and received by the eye at the 

 distance of 10 to 12 feet through a prism of flint glass, free 

 from veins, held near the eye, the beam is seen to be separated 

 into the four following colours only, red, yellowish green, blue, 

 and violet, in the proportions represented in fig. 2. 



" The line A that bounds the red side of the spectrum is 

 somewhat confused, which seems in part owing to want of 

 power in the eye to converge red light. The line B, between 

 red and green, in a certain position of the prism, is perfectly 

 distinct ; so also are D and E, the two limits of violet. But 



