16 
NATURE 
[May 4, 1871 

No anatomist has hitherto been able to distinguish these three 
systems of nerves by microscopic observation. But it is admitted 
in physiology that the only way in which the sensation excited 
by a particular nerve can vary is by degrees of intensity. The 
intensity of the sensation may vary from the faintest impression 
up to an insupportable pain ; but whatever be the exciting cause, 
the sensation will be the same when it reaches the same intensity. 
If this doctrine of the function of a nerve be admitted, it is 
legitimate to reason from the fact that colour may vary in three 
different ways, to the inference that these three modes of varia- 
tion arise from the independent action of three different nerves 
or sets of nerves. 
Some very remarkable observations on the sensation of colour 
have been made by M. Sigmund Exner in Prof, Helmholtz’s 
physiological laboratory at Heidelberg. While looking at an 
intense light of a brilliant colour, he exposed his eye to rapid 
alternations of light and darkness by waving his fingers before 
his eyes. Under these circumstances a peculiar minute structure 
made its appearance in the field of view, which many of us may 
have casually observed. M. Exner states that the character of 
this structure is different according to the colour of the light em- 
ployed. When red light is used a veined structure is seen ; when 
the light is green, the field appears covered with minute black 
dots, and when the light is blue, spots are seen, of a larger size 
than the dots in the green, and of a lighter colour. 
Whether these appearances present themselves to all eyes, and 
whether they have for their physical cause any difference in the 
arrangement of the nerves of the three systems in Helmholtz’s 
theory I cannot say, but I am sure that if these systems of nerves 
have a real existence, no method is more likely to demonstrate 
their existence than that which M. Exner has followed. 
CoLour BLINDNESS 
The most valuable evidence which we possess with respect to 
colour vision is furnished to us by the colour-blind. A con- 
siderable number of persons in every large community are unable 
to distinguish between certain pairs of colours which to ordinary 
people appear in glaring contrast. Dr. Dalton, the founder of 
the atomic theory of chemistry, has given us an acccount of his 
own case. 
The true nature of this peculiarity of vision was first pointed 
out by Sir John Herschel in a letter written to Dalton in 1832, 
but not known to the world till the publication of ‘‘ Dalton’s 
Life” by Dr. Henry. The defect consists in the absence of one 
of the three primary sensations of colour. Colour-blind vision 
depends on the variable intensities of two sensations instead of 
three. The best description of colour-blind vision is that given 
by Prof. Pole in his account of his own case in the ‘ Phil. 
Trans.,” 1859. 
In all cases which have been examined with sufficient care, the 
absent sensation appears to resemble that which we call red. 
The point P on the chart of the spectrum represents the relation 
of the absent sensation to the colours of the spectrum, deduced 
from observations with the colour box furnished by Prof. Pole. 
If it were possible to exhibit the colour corresponding to this 
point on the chart, it would be invisible, absolutely black, to 
Prof. Pole. As it does not lie within the range of the colours of 
the spectrum we cannot exhibit it; and, in fact, colour-blind 
people can perceive the extreme end of the spectrum which we 
call red, though it appears to them much darker than to us, and 
does not excite in them the sensation which we call red. In 
the diagram of the intensities of the three sensations excited by 
different parts of the spectrum, the upper figure, marked P, is 
deduced from the observations of Prof. Pole; whi'e the lower 
one, marked K, is founded on observations by a very accurate 
observer of the normal type. 
The only difference between the two diagrams is that in the 
upper one the red curve is absent. The forms of the other two 
curves are nearly the same for both gbservers. Ve have great 
reason therefore to conclude that the colour sensations which 
Prof. Pole sees are what we call green and blue. This is the 
result of my calculations ; but Prof. Pole agrees with every other 
colour-blind person whom I know in denying that green is one 
of his sensations. The colour-blind are always making mistakes 
about green things and confounding them withred. The colours 
they have no doubts about are certainly blue and yellow, and 
which they are able to see. 
To explain this discrepancy we must remember that colour- 
blind persons learn the names of colours by the same method as 


ourselves. They are told that the sky is blue, that grass is green. 
that gold is yellow, and that soldiers’ coats are red. They observe 
difference in the colours of these objects, and they often suppose 
that they see the same colours as we do, only not so well. But 
if we look at the diagram we shall see that the brightest example 
of their second sensation in the spectrum is not in the green, but 
in the part which we call yellow, and which we teach them to 
call yellow. The figure of the spectrum below Prof. Pole’s curves 
is intended to represent to ordinary eyes what a colour-blind 
person would see in the spectrum. I hardly dare to draw your 
attention to it, for if you were to think that any painted picture 
would enable you to see with other people’s vision I should 
certainly have lectured in vain. 
ON THE YELLOW Sror 
Experiments on colour indicate very considerable differences 
between the vision of different persons, all of whom are of the 
ordinary type. A colour, for instance, which one person on 
comparing it with white will pronounce pinkish, another person 
will pronounce greenish, This difference, however, does not 
arise from any diversity in the nature of the colour sensations in 
different persons. It is exactly of the same kind as would be 
observed if one of the persons wore yellow spectacles. In fact, 
most of us have near the middle of the retina a yellow spot 
through which the rays must pass before they reach the sensitive 
organ : this spot appears yellow because it absorbs the rays near 
the line F, which are of a greenish-blue colour. Some of us 
have this spot strongly developed. My own observations of the 
spectrum near the line F are of very little value on this account. 
Iam indebted to Professor Stokes for the knowledge of a method 
by which any one may see whether he has this yellow spot. It 
consists in looking at a white object through a solution of chloride 
of chromium, or at a screen on which light which has passed 
through this solution is thrown. This light is a mixture 
of red light with the light which is so strongly absorbed by 
the yellow spot. When it falls on the ordinary surface of the 
retina it is of a neutral tint, but when it falls on the yellow spot 
only the red light reaches the optic nerve, and we see a red spot 
floating like a rosy cloud over the illuminated field. 
Very few persons are unable to detect the yellow spot in this 
way. The observer K, whose colour equations have been used 
in preparing the chart of the spectrum, is one of the very few 
who do not see everything as if through yellow spectacles. As 
for myself, the position of white light in the chart of the spectrum 
is on the yellow side of true white even when I use the outer 
parts of the retina ; but as soon as I look direct at it, it becomes 
much yellower, as is shown by the point W C. It is a curious 
fact that we do not see this yellow spot on every occasion, and 
that we do not think white objects yellow. But if we wear spec- 
tacles of any colour for some time, or if we live in a room lighted 
by windows all of one colour, we soon come to recognise white 
paper as white. This shows that it is only when some alteration 
takes place in our sensations that we are conscious of their 
quality. 
There are several interesting facts about the colour sensation 
which I can only mention briefly. One is that the extreme parts 
of the retina are nearly insensible to red. If you hold a red 
flower and a blue flower in your hand as far back as you can see 
your hand, you will lose sight of the red flower, while you still 
see the blue one. Another is, that when the light is diminished - 
red objects become darkened more in proportion than blue ones. 
The third is, that a kind of colour blindness in which blue is the 
absent sensation can be produced artificially by taking doses of 
santonine. This kind of colour blindness is described by Dr. 
Edmund Rose, of Berlin. It is only temporary, and does not 
appear to be followed by any more serious consequences than 
headaches. I must ask your pardon for not having undergone a 
course of this medicine, even for the sake of becoming able to 
give you information at first hand about colour-blindness. 
J. CLERK MaxwWeELi 


SCIENTIFIC SERIALS 
Tue Quarterly Journal of Science for April commences with 
a yery interesting account, by Dr. Hofmann, of the early days of 
| the Royal College of Chemistry, under the title of ‘‘A Page of 
they persist in saying that yellow, and not green, is the colour | 
Scientific History.” After tracing the influence of Liebig’s 
school at Giessen on the progress of chemical science in this 
country, and the choice of himself, at the recommendation of 
Liebig, as the professor at the laboratory which it ‘was deter- 
