THE THEOEY OE COMPOUND COLOUES. 
77 
time, when we shall find that we judge white objects to be white, in spite of the rays 
which enter the eye being coloured. 
Now ordinary white fight is a mixture of all kinds of fight, including that between E 
and F, which is partially absorbed. If, therefore, we compound an artificial white con- 
taining the absorbed ray as one of its three components, it will be much more altered 
by the absorption than the ordinary fight, which contains many rays of nearly the same 
colour, which are not absorbed. On the other hand, if the artificial fight do not contain 
the absorbed ray, it will be less altered than the ordinary fight which contains it. 
Hence the greater the absorption the less green will those colours appear which are 
near the absorbed part, such as (48), (52), (56), and the more green will the colours 
appear which are not near it, such as (32), (36), (40). And these are the chief differences 
between fig. 4 and fig. 5. 
I first observed this peculiarity of my eyes when observing the spectrum formed by a 
very long vertical slit. I saw an elongated dark spot running up and down in the blue, 
as if confined in a groove, and follomng the motion of the eye as it moved up or down 
the spectrum, but refusing to pass out of the blue into other colours. By increasing the 
breadth of the spectrum, the dark portion was found to correspond to the foramen 
centrale, and to be Hsible only when the eye is turned towards the blue-green between E 
and F. The spot may be well seen by first looking at a yellow paper, and then at a blue 
one, when the spot will be distinctly seen for a short time, but it soon disappears when 
the eye gets accustomed to the blue'*'. 
I have been the more careful in stating this peculiarity of my eyes, as I have reason 
to believe that it affects most persons, especially those who can see Haidixger’s brushes 
easily. Such persons, in comparing their vision with that of others, may be led to think 
themselves affected with partial colour-blindness, whereas their colour-vision may be of 
the ordinary kind, but the rays which reach their sense of sight may be more or less 
altered in their proportions by passing through the media of the eye. The existence of 
real, though partial colour-blindness will make itself apparent, in a series of observations, 
by the discrepancy between the observed values and the means being greater in certain 
colours than in others. 
§ XVI. General Conclusions. 
Neither of the observers whose results are given here show any indications of colour- 
blindness, and when the differences arising from the absorption of the rays between 
E and F are put out of account, they agree in pro-ving that there are three colours in the 
spectrum, red, green, and blue, by the mixtures of which colours chromatically identical 
with the other colours of the spectrum may be produced. The exact position of the red 
and blue is not yet ascertained ; that of the green is \ from E towards F. 
The orange and yellow of the spectrum are chromatically equivalent to mixtures of 
red and green. They are neither richer nor paler than the corresponding mixtures, and 
the only difference is that the mixture may be resolved by a prism, whereas the colour 
* See tEe Eeport of the British Association for 1856, p. 12. 
MDCCCLX. 
M 
