| 252 A. M. Mayer—History of Young's 
experiment; and that these experiments then vindicated his 
hypothesis and raised it to the dignity of a theory.* 
Before discussing the subject proper of this article, it may be 
well to give the reader a clear conception of Young’s theory of 
color, and to show in what high estimation it is at present held 
y men of science. This can best be done by the reading of 
the following short extracts from Helmholtz’s “ Physiological 
Optics” and from his ‘t Popular Scientific Lectures.” 
“To speak of three fundamental colors in an objective sense 
found in the physiological study of colors. Young states that: 
“J, There exist in the eye three kinds of nerve fibers whose 
excitation respectively gives the sensation of red, of green, and 
‘““*2. Homogeneous light excites the three kinds of nerve 
fibers with an intensity which varies with its wave-length.’ 
That which possesses the greatest length of wave excites most 
powerfully the fibers sensitive to red, that which has an average 
wave-length excites the nerves sensitive to green, while that 
light formed of the shortest waves acts on the fibers which give 
the violet sensation. Nevertheless we cannot deny, but rather 
should admit for the explanation of numerous phenomena, that 
each color of the spectrum excites all three kinds of nerve 
fibers, but with different intensities. Imagine the colors of the 
spectrum arranged horizontally in going from the red, R, to the 
violet, V, as shown at the base of figure 1. The three curves 
will then represent more or less exactly the degrees of irrita- 
bility of the three kinds of nerve fibers (1, the red; 2, the 
green; 3, the violet) for the various colors of the spectrum. 
* These two terms, hypothesis and theory, are so generally misunderstood and 
thoughtlessly used that it may be well here to give two concise definitions; the 
first is by Flourens; the second is by Prof. J. Henry. 
“ An hypothesis is the explanation of facts by possible causes ; a theory is the ex- 
“A supposition or guess thus made from analogy as to the nature of the law of 
bility. When an hypothesis of this kind has been extended and verified, or, im 
other words, when it has become an exact expression of the law of a class of facts, 
it called a theory.” 
