126 Proceedings of Royal Society of Edinburgh. [sess. 
Dr Ivonig has been one of those who, although a standi supporter 
of the Young-Helmholtz theory, has openly dissented from its 
original application to colour-blindness. In the inquiry of 1883, he 
explicitly stated that, according to recent discoveries, it had been 
proved that dichromatisin could not he explained by the assumption 
that one of the fundamental colours was absent, and the other two 
were in action. And in 1886, in his communication to the British 
Association, he stated the same thing, admitting that the colours 
were yellow and blue. But, although holding this opinion, he made 
a most elaborate investigation to test the applicability of the rejected 
explanations. In this he was joined by Dr Dieterici, and the 
results were given in a paper presented to the Berlin Academy on 
“ The Fundamental Colour-Sensations, and the Distribution of their 
Intensity in the Spectrum.” The paper was of so much importance 
that it is desirable to give some account of it. 
The authors began by stating that the first step necessary was to 
analyse the series of spectral colours by reducing them to a 
minimum number of “ elementary sensations,” the combinations of 
which, in different proportions, would give all the spectral hues. 
For this purpose they chose three — one near the red end, one near 
the violet end, and one in the green, distinguishing these 
elementary sensations as R, V, and G respectively. Then they had, 
by proper instrumental experiments, to find the proportions of each 
that would give the various hues, and finally to draw the “ ele- 
mentary sensation curves ” representing the colour vision of different 
persons. All this had been done before by Clerk Maxwell, 
Donders, and Van der Weyde, but the new investigators did it with 
more care, and with a special view to further researches later on. 
They tried in this way three kinds of vision — viz., total 
colour-blindness ; dichromic vision ; and normal vision. The 
dichromic interests us most here. They tested the so-called “ green- 
blind ” and “ red-blind,” and their results agreed tolerably well with 
those of Van der Weyde. The warm curve for both classes was 
nearly similar in form, but for the green-blind it was situated 
nearer the red end of the spectrum, its maximum being at 570, 
whereas the maximum for the red-blind was about 550 to 555. 
The neutral points, were about 502 for the green and 492 for the 
red-blind, but were indistinct and difficult to decide on. The cold 
