Discovery of his Theory of Colors. 265 
Young then replaces Wollaston’s “ yellowish green” by ‘‘green,” 
and farther on he adds, ‘The colours differ scarcely at all in 
quality within their respective limits, but they vary in bright- 
that both Young and Wollaston were of the opinion that 
when a narrow bright crevice is observed through a prism, 
that the spectrum so viewed consists of only four colors, red, 
green, blue and violet, “differing scarcely at all in quality 
within their respective limits ;’ which limits they supposed nat- 
urally existed in the dark spaces which, as they imagined, 
bounded these elementary colors. Young, however, somewhat 
modifies this opinion in the next sentence, when he says: “A 
narrow line of yellow is generally visible at the limit of the red 
and green, but its breadth scarcely exceeds that of the aperture 
by which the light is admitted, and Dr. Wollaston attributes 
it to the mixture of the red with the green light.” It would, in- 
deed, appear from the last portion of this sentence that Young 
obtained directly from Wollaston one of the main facts on 
which his theory was founded, namely, that yellow can be re- 
produced by the mixture of red.and yellow lights. But Wol- 
laston, in his paper of 1802, from which we have cited, makes 
no such 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 
observation on the color composition of the spectrum is the 
correct one; yet the errors of observation of Wollaston and of 
oung were errors which led toa t discovery, as we shall 
see on the further examination of the history of this beautiful 
and comprehensive theory of color. 
Farther on in the Natural Philosophy we read that, “ The sen- 
sations 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 combination of tints, result- 
ing from the mixture of the colours.” These experiments were 
him, were destined to remain unnoticed, “until a later genera- 
tion, by slow degrees, arrived at the discovery of his discovery.” 
