128 Proceedings of Boy al Society of Edinhurgli. [sess. 
And it was found that “ the two types of dichromic vision 
hitherto investigated might he considered to arise out of the normal 
trichromic system, in such a way that in one type the fundamental 
red sensation, and in the other type the fundamental green sensa- 
tion, were wanting.” 
Diagrams illustrating these fundamentals are copied in Helmholtz, 
pages 340 and 370. 
The authors remarked that the three sensations, thus found, corre- 
sponded with those which Hering, by a pure physiological analysis, 
had designated as “ Urroth^ Urgriin, and Urhlauf and that the 
complement to the blue sensation was his Urgelh of iv.l. 575. 
They also pointed out the possible coincidence of the red and 
green curve, producing the dichromic yellow; they called this 
“ the modern development of Thomas Young’s theory of colour- 
blindness,” and took credit for the proof it afforded that “ the 
views of Thomas Young, slightly modified by modern experimental 
research, were perfectly correct; and that in this way modern 
science seemed to have breathed into his theory of colour vision a 
life of such vigour that it would flourish for ever ! ” 
Their results were afterwards modified by Helmholtz ; hut, never- 
theless, their labours were very valuable, and contributed materially 
to his later solution of the problem. 
AD. 
Ilillebrand on the Specific Luminosity of Colours. 
This is an elaborate essay on a complicated and difficult subject, 
viz., the connexion of luminosity and colour. After an intro- 
duction by Hering, alluding to some changes of his own views, the 
author, in a long chapter, discusses the nature of the subject, 
and the import of several terms used (Helligkeit, Lichtstarke, 
Intensitat, Sattigung, &c.). He then, after mentioning the previous 
investigations by Fraunhofer, Vierordt, Briicke, Mac^, and Mcati, 
explains his own viev;s and objects. He considers that the total 
“ Helligkeit ” of any spectral colour is made up of two factors : — 
First the power which the colour possesses of exciting the sensation 
of white ; and secondly, a “ specific luminosity ” of the colour 
itself, varying for different colours. He has found means of 
