256 
NATURE 
| Fuly 28, 1870 
Microscope, in NAtuRE, No. 37. It is, I believe, quite a mis- 
take to say that you cannot get a cheap working English 
instrument, The model of Crouch, of London Wall, for 
instance, is not very much dearer than Hartnack’s small model, 
and yet, at the same time, is in every way better and more com- 
fortable to work with. Crouch’s rackwork is so good that tor 
iths and iths there is no need to resort to the fine adjustment, 
except on special occasions, whereas, with the sliding tube, the 
fine adjustment is so continually used that it is very soon 
thrown wrong, to say nothing of the trouble which a beginner 
has in working the sliding tube successfully. I once had some 
of Nachet’s instruments, to which Hartnack’s are very like, in 
use in my class at University College, and the sliding movement 
was very successful in smashing my best specimens and injuring 
the front surfaces of my lenses. Then again, the English 
length of tube is undoubtedly an advantage for a slanting posi- 
tion of the microscope, and I suppose that is by far the most 
common position in which an instrument is used. In addition, 
Crouch’s instrument affords an admirably effective but simple 
stage movement, which may be entirely removed at pleasure, 
and has a simple sub-stage tube, into which, if required (and 
in London it often becomes a necessity), a condenser might be 
fitted. Crouch’s instrument is like Hartnack’s, simple but 
strong, steady, and cheap ; it differs in being about three times 
as convenient, and will, probably, last twice as long. 
Nor can I agree with Mr. Lankester in recommending the 
general purchase of Hartnack’s glasses for student’s use. His 
No. 8, for instance, in many respects an admirable glass, is 
terribly close, and for this, and apparently for other reasons, 
very soon gets spoilt when used by students. I have been using 
lately for my classes at University College, a ¢th of Crouch’s, 
which, as far as ordinary histological work is concerned, performs 
in the most satisfactory manner, and yet is quite a cheap glass. 
I suppose the question of the price of labour prevents our 
English manufacturers from bringing down their prices to quite 
the French level, but I believe we get quite an equivalent for the 
slight excess in the form of greater convenience and better work- 
manship. M. Fosrer 
Colour-Blindness 
THE nature of colour-blindness has never, I think, been satis- 
factorily ascertained. The usual explanation appears to be 
that the eye of the colour-blind is insensible, or nearly insensible 
to light of some particular colour. This I regard as in many 
respects unsatisfactory, 2nd as Iam not aware that the theory 
which I now suggest has been advocated before, I venture to 
lay it before the public. 
There are no doubt some cases in which the eye seems partially 
insensible to particular colours or to colours in general. In such 
cases, however, I believe there is usually defective vision, and 
not proper colour-blindness. Those only are to be regarded as 
truly colour-blind who can perceive figures distinctly, but con- 
found colours which other persons distinguish. Such was the 
case, for example, with Dugald Stewart, who could not dis- 
tinguish between the colour of the leaves and the fruit of a 
Siberian crab; but he saw both, and therefore could distinguish 
the colour of both from that of the sky or cloud which lay beyond 
them. I mention this case more especially because Stewart was 
a psychologist, and maintained in opposition to Reid that variety 
of colour is the means by which we perceive visible figure. This 
is at least conclusive as to the perception of variety of colours by 
the colour-blind, which all observations made upon them point to. 
Many philosophers have attempted to explain the phenomenon 
by assuming an insensibility to some colour, red for example. 
My reasons for rejecting this explanation are: (1) that in 
some cases where the experiment was tried (see Prof. Wart- 
mann’s paper in the Scientific Memcirs for November 1844) 
the colour-blind Zperson saw the whole of the visible 
spectrum ; and (2) that if red (for example) were seenas black, 
there would be no danger of confounding it with green, which 
would, on this hypothesis, be the colour seen most distinctly ; 
but, in fact, confusion in regard to one colour almost always 
extends to the complementary tint. 
The explanation I would offer is that derived from seeing 
accidental or complementary colours. It isa known fact that the 
eye has, in general (whether natural or acquired), a peculiar 
aptitude for white light, and thus if I gaze on a bright surface 
of any other colour, and look away rapidly towards a dark 
ground, the complementary hue becomes immediately visible. 
Nor can it, I think, be doubted that the complementary hue is 
not produced by the act of looking away. Green, for example, 
is produced by the red light falling on the eye, and the effect of 
looking away towards the dark ground is merely to make the 
green separately visible by cutting off the supply of red. It 
previously coexisted with and modified the red, but in ordinary 
eyes only to such an extent as not to prevent the red fromstrongly 
predominating in the total perception. This coexistence of the 
complementary colour with that actually visible is, I believe, 
known to persons accustomed to make delicate experiments in 
optics. I recollect in some lectures on the subject which I 
attended two or three years ago (where the equality of two lights 
of slightly different colours had to be determined), the professor 
cautioned us against looking too long at the lights, as he always 
found in his own case that there was a change of shade anda 
consequent impairment of the accuracy of his determination if he 
did so. This I have no donbt arose from the cause I have 
intimated. When we bear in mind the mutual excitation 
of sound-vibrations, the fact will create no astonishment. 
Now I apprehend that in most instances true colour-blind- 
ness arises from these complementary colours being excited 
more rapidly and with greater intensity than in ordinary eyes. 
If, for example, on Jooking at a red object the complementary 
green was excited almost at once, and with such intensity as 
materially to modify the red ; and if, on the other hand, on looking 
at a green object, the excitation of the complementary red took 
place with equal readiness and intensity, it is clear that such an 
eye could not distinguish red from green. Both colours would, 
in fact, be seen after the first instant as a white or grey. In con- 
firmation of this view I may remark that, according to Seebeck, 
all the colour-blind persons whom he examined confounded the 
colours with grey. Another argument inits favour is, that a con- 
fusion in regard to one colour seems (according to Wartmann) 
always to extend to the complementary tint. Again, it is natural 
to suppose that the production of complementary colours will 
take place rapidly and with considerable intensity when the eye 
is unusually sensitive to the incident light. Now, I find this 
unusual sensiliveness noticed in several of Wartmann’s examples. 
One young woman could read for nearly a quarter of an hour 
(in the evening) after any one else could. In the cases men- 
tioned by Goethe, the sight of the young men was ‘‘ very good,” 
and they ‘‘appreciated with great delicacy the gradations of 
light and dark.” ‘* Many Daltonians,” says Wartmann, ‘‘see 
better in a demi-obscurity than other persons whose sight is more 
piercing by day than theirs,” which he goes on to say was the 
case with three whom he himself had examined. Lastly, from the 
same paper it would appear that the colour-blind are either 
insensible tothe phenomenon of accidental (complemental) colours, 
or see it with great difficulty. This, of course, is just as it should 
be on my theory. The colour-blind man sees both colours while 
looking at the coloured object, and he will again see both on 
looking away from it at the dark ground. If, for example, the 
colour looked at be red, the accidental green is seen while looking 
at the red, and it is also vivid enough to produce a secondary 
accidental red on looking away. The change produced by 
looking away will, therefore, be very slight, and hardly, if at all, 
perceptible. 
I do not, however, put this theory forward as a complete 
explanation even of true colour-blindness. In addition to 
accidental ‘or ‘complemental colours, I believe there is often 
another phenomenon which may be called subjective colours, 
which modifies the total perception. In jaundice, it is well 
known, that black objects will appear yellow, and Dr. Wartmann 
records one case in which black appeared to the eye of the patient 
as green or crimson. I may have something more to say on this 
point hereafter. In such eyes, in fact, the adaptation is not for 
white light, but for light of some other colour, and the whole 
phenomenon of accidental colours is altered aecordingly. 
If these views be correct, it is evident that the colour-blind 
man will be best able to discriminate colours when he merely 
takes one glance at the coloured object, and then looks away 
towards a dark surface. This is worth trying. The fact that 
form is most easily discerned by taking a pretty long look at the 
object, makes a man follow the same course when he wishes to 
discriminate colours, but the advisability of doing so may be 
doubted. Another consequence is, that the colour-blind man 
would probably discriminate colours more readily in a faint light 
than in a bright one. These two observations can be easily made, 
and if my prediction should prove correct, the result will be of 
practical advantage to the colour-blind as well as a confirmation 
of the theory. 
Trinity College, Dublin, July 9 W. H. S. Monck 
