112 Prof. A. M. Mayer on the History of Young's 



ration of the dark lines in the solar spectrum, and also on the 

 basis of an erroneous observation made by Young in repeat- 

 ing Wollaston's experiment ; 4th, that Young subsequently 

 tested his hypothesis of colour-sensation and found that it was 

 in accord with facts reached by 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 colour-sensations, and to show in what high estimation it is 

 at present held by men of science. This can best be done by 

 the reading of the following short extracts from Helmholtz's 

 ' Physiological Optics ' and from his ' Popular Scientific 

 Lectures.' 



" To speak of three fundamental colours in an objective 

 sense would be nonsense ; in fact, as long as one refers only 

 to purely physical conditions, and while there is no reference 

 to the human eye, the properties of compound light depend 

 alone on the proportions in which exist lights of different 

 wave-lengths. The reduction of light to three fundamental 

 colours can never have any thing else than a subjective signi- 

 fication ; it consists simply in reducing all colour-sensations to 

 three fundamental sensations. It is in this sense that Young 

 understood the problem; and his hypothesis gives, in fact, an 

 exceedingly clear and simple explanation and summary of all 

 of the phenomena found in the physiological study of colours. 

 Young states that — 



" ' 1. There exist in the eye three kinds of nerve-fibres, 

 whose excitation respectively gives the sensation of red, of 

 green, and of violet. 



" ' 2. Homogeneous light excites the three kinds of nerve- 

 fibres with an intensity which varies with its wave-length.' 

 That which possesses the greatest length of wave excites most 

 powerfully the fibres sensitive to red, that which has an ave- 

 rage wave-length excites the nerves sensitive to green, while 

 that light formed of the shortest wave acts on the fibres which 

 give the violet sensation. Nevertheless we cannot deny, but 

 rather should admit for the explanation of numerous pheno- 

 mena, that each colour of the spectrum excites all three kinds 

 of nerve-fibres, but with different intensities. Imagine the 

 colours of the spectrum arranged horizontally, going from 

 the red, R, to the violet, V, as shown at the base of fig. 1. The 

 three curves will then represent more or less exactly the de- 

 grees of irritability of the three kinds of nerve-fibres (1, the 

 red ; 2, the green ; 3, the violet) for the various colours of the 

 spectrum. 



