Discovery of his Theory of Colours. 121 



we at once recognize as the dark solar line D, occupying a 

 position between the orange and yellow, and well known to be 

 caused by the reversal of the bright yellow light of sodium 

 vapour. No one, however, could now say, after an exami- 

 nation of the spectrum as observed by Wollaston, that the 

 line D divides the red from the green. Wollaston also calls 

 his D and E lines (the G and H lines of Fraunhofer) " the 

 two limits of the violet ;" we now know that G is really on the 

 indigo, and that H is within the limits of the violet. 



For a more satisfactory comparison of the colours of the 

 solar spectrum as observed by Wollaston and Fraunhofer, I 

 give below the following Table. Fraunhofer's results are 

 taken from his coloured figure of the spectrum. Both spectra 

 are from flint glass, and their lengths are supposed divided 

 into 360 equal parts. 





Fraunhofer. 



Wollaston. 



Red . 



. . 56 



57-6 Red. 



Orange 

 Yellow 



. . 27 

 . . 27 





Green . 



. . 46 



82*8 Yellowish green 



Blue . 



. . 48 



129-6 Blue. 



Indigo . 



. . 47 





Violet . 



. . 109 



90 Violet, 



360 360 



Fraunhofer's observations are irreproachable, and are to this 

 day in high esteem for their accuracy. They were made by 

 placing the prism in front of a telescope mounted on a divided 

 horizontal circle, and viewing a distant slit through the prism 

 and telescope. He observed spectra as pure as those given by 

 modern spectroscopes of low power. Fraunhofer discerns 

 orange and yellow and green where Wollaston only sees yel- 

 lowish green. Also Wollaston did not see all of the violet, as 

 we might suspect from his having bounded its upper limits by 

 the line H. Fraunhofer saw 109 parts of violet, Wollaston 

 only 90. 



The above discussion, I think, has clearly shown that Wol- 

 laston made a false interpretation of his observation in suppo- 

 sing that he had discovered a pure spectrum naturally divided 

 by dark lines into four simple colours, and that he also erred 

 in the relative proportions which he gave to them. Also I 

 have shown that Young, in finally selecting red, green, and 

 violet as the three elementary colour-sensations, was not, as 

 Helmholtz states, guided in their choice " by the consideration 

 that the extreme colours of the spectrum occupied the privi- 

 leged positions," but selected those colours on hearing of Wol- 



PhU. Mag. S. 5. Vol. 1. No. 2. Feb. 1876. K 



