COLOUR-BLINDNESS. 
499 
colour depend, and that it is not a part of the object itself. 
The meaning of this will be best understood by an example. 
When a ray of light falls on the green grass, part of the ray 
is absorbed and part reflected, and the grass is only seen with 
the part that is reflected. The green we see consists of the 
original white light, deprived of a portion of its rays by absorp- 
tion. It is, therefore, partial darkness, and not absolute light, 
consequently not a pure and absolute green, but only a residual 
group of the unabsorbed coloured rays. A poppy appears 
scarlet, as it absorbs all the colours of the rays except red, 
and hence its peculiar tint ; but if it be looked at through 
green glass it will appear black : as the poppy only reflects 
the red ray, this is absorbed by the green glass. The red of 
the rose, the blue of the violet, the yellow of the jonquil, are 
due to their absorption of all the rays excepting the red, blue, 
and yellow. The pale-tinted rose, almost white, reflects nearly 
all the coloured rays. We can, therefore, easily perceive, 
without light, the face of nature would be that of a world in 
mourning ; it is light that enlivens the scene, painting the 
exterior with a beauty, richness, delicacy, and harmony that 
man vainly attempts to rival. Colour is so dependent on 
light, that when artificially produced, as by candle or gas, from 
not being pure, many things appear of a different colour, as is 
well known by the lady who attempts to choose a ribbon or 
the artist who paints a picture by artificial light : a blue being 
mistaken for a green, and a green for a blue. On a moonlight 
night we cannot distinguish the colour of a chimney-pot ; and 
were we to take a number of pieces of cloth, or different 
coloured papers, and examine them by the bright light of the 
moon, and write on the back of each the colour it appears, we 
should be astonished in daylight to see how much we had been 
deceived as to the true tint of each. 
Assuming, therefore, that the sound eye can see perfectly 
well three simple colours — red, yellow, and blue — and that all 
the rest of the colours of the spectrum are mixtures of these 
with each other, let us now proceed to inquire what is the 
peculiar condition of sight in those persons who, being unable 
to distinguish certain rays, are, as we have already stated, 
colour-blind ; but not necessarily owing to disease of the optic 
nerve or retina, but simply arising from inability to recognize 
those rays of light which consist of pure red. 
Professor Maxwell, who has closely and philosophically 
investigated the subject, says : — “ The mathematical expres- 
sion of the difference between the colour-blind and ordinary 
vision is, that colour to the former is a function of two inde- 
pendent variables, but to an ordinary eye of three ; and that 
the relation of the two kinds of vision is not arbitrary, but 
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