Before considering the subject of the extinction of light by other
types of colour vision, attention must be called to what has already
been brought before you. The various colours of the spectrum have to
be reduced to the following amounts before they suffer extinction, the
orange light at D being of the value of one candle. (See appendix, page
217, for complete tables.)
Reduction in
Millionths. Remarks.
B 10,000 or 1/100 approximately pure red
sensation
C 1,100 or 1/909 rather more scarlet
D 50 or 1/20000 orange light
E 6·5 or 1/154000 a green chosen by Maxwell
as a standard colour
F 15·0 or 1/67000 beginning of the blue
Blue Lithium 85·0 or 1/11700 a good sample of blue
G 300·0 or 1/3300 approximately pure sensation
of violet.
If we make these same colours all of the luminosity of one amyl-acetate
lamp (·8 of a candle), we find that the numbers are as follows:--
Reduction in
Millionths.
B 300
C 225
D 48
E 3·3
F ·9
Blue Lithium 1·1
G 1·1
These numbers are remarkable, and we may enforce what they mean in
this way. The energy of radiation, and of light also when of ordinary
luminosity, varies inversely as the square of the distance from an
incandescent body when of small dimensions. But from the above it seems
that a white screen receiving the rays from an amyl-acetate lamp in an
otherwise perfectly dark place, and having a colour which stimulates
the red sensation alone, would be invisible at 58 feet distance, for
there would not be enough energy transmitted to stimulate the red
perceiving apparatus sufficiently to give the sensation of light. If
it were an orange light, such as sodium, of the same luminosity, we
should have to move it from the screen 142 feet before the same result
was attained. With the green light at E, the distance would be 550
feet, and with the violet the distance would be increased to 1000 feet.
The reduction in intensity of white light, which, when of ordinary
brightness, is warm, would make it colder, for the red would disappear,
and finally the residue of light, just before extinction, would become
a cold grey, due to the absence of all colour. The changes in hue that
would occur are variable, the variation being due to the loss of colour
of the different rays for different amounts of reduction, and then
their final extinction. We can place two patches of white light on the
screen, and gradually reduce one in intensity, keeping the other of its
original value. No one would expect that the two would be dissimilar
in hue, as they appear to be when the former is moderately near the
extinction value. If we wish to see this perfectly, we should use an
extinction box and view it away from the surroundings, which must be
more or less slightly illuminated.
It has already been stated that the persistency curve for persons who
have normal colour vision is closely the same as that recorded for
those who are of the monochromatic type. As this is so, we must expect
to find that the persistency curve of these last is the same as their
luminosity curve. We put this to the test of experiment and found that
our reasoning was correct, for the persistency curve could be almost
exactly fitted to it. (See table, pages 217 and 222.) The slight
difference between them can be credited to the fact that the whole eye
may have been brought into use during the extinction observations, the
centre of the eye not being exclusively used. The Figure 31 shows
both the extinction and the persistency curves, and also the curve of
luminosity for the normal eye.
[Illustration: FIG. 31.]
The former were derived from a case P. sent for examination. P. and
Q. are brothers, each of whom possesses but one colour sensation, and
examination showed that their vision was identical. Mr. Nettleship
has kindly given me the following particulars regarding them:--“Their
acutes of vision (form vision) in ordinary daylight is only one-tenth
of the normal. A younger sister and brother are idiotic and almost
totally blind, and in one of these the optic nerves show clear
evidence of disease. Hence, the colour blindness of P. and Q. must
almost without doubt be considered as the result of disease, perhaps
ante-natal, involving some portion of the visual apparatus.” A lack
of acuteness of vision would be expected from the small amount of
light they perceive compared with normal vision. The fact that two of
a family, not twins, possess exactly the same colour sense, and that
their extinction curves are entirely different to those suffering from
post-natal disease, but similar to those of normal vision, point to
their colour blindness as falling in the same general category as that
of the congenital type. To this I shall refer again.
[Illustration: FIG. 32.]
We may reason still further. With the red- and green-blind the violet
sensation is still present, and we may therefore expect that their
extinction curves, and consequently their persistency curves, should
be alike, and should also agree with that made from your lecturer’s
observations. A study of Figures 32 and 33 will tell you that such is
practically the case. The former shows the luminosity, the persistency,
and the extinction curves of a completely red-blind subject, and the
latter the same curves for a green-blind subject (see pages 223 and
224). Both were excellent observers, and their examination was easy,
owing to the acquaintance with scientific methods. The accuracy of
their results may be taken as unquestionable. Each of them may be taken
as a representative of their own particular type of colour blindness.
There is an agreement between them at the violet end, but a deviation
at the red end of the spectrum. The general form of the curves
indicates that the same sensation is extinguished last in all. Now,
have we any other criterion to offer? We have. In the first instance,
we have the violet-blind person to compare with the others, and also
another observer who had monochromatic vision, but whose sensation
was different to that of the two monochromatic cases we have so far
brought to your notice. We have already stated the peculiarities in
colour nomenclature of the violet-blind case. His curve of luminosity
for the spectrum was taken (page 227), and when compared with the
curve of normal luminosity, it became evident that in the red and up
to the orange his measures were those which a normal eye would make;
but that the luminosity fell off in the green, and finally disappeared
to an immeasurable quantity in the violet (see Fig. 30, curves M and
F). If his measures of spectrum luminosity are deducted from those of
the normal eye, and the ordinates be increased proportionately to make
the maximum difference 100, the figure so produced, when compared with
the _luminosity_ curve obtained from the monochromatic observers, was
found to be the same, and consequently with the persistency curves
above referred to. Endeavours were made to gain a good extinction
curve, but the results were not as successful as could be desired; but
it was ascertained that, without doubt, his most persistent sensation
was not more than 1/180 as lasting as that of the normal eye, or to put
it in another way, his green at E was only extinguished when the energy
falling on his eye was 180 times greater than that at which it vanished
with the normal eye. This plainly teaches us that the missing sensation
was that which, when present, is ordinarily the most persistent.
The next is a case of monochromatic vision, which differs from those
previously brought before you, and I cannot do better than describe it
in the words which General Festing and myself employed in our paper in
the “Philosophical Transactions.”
[Illustration: FIG. 33.]
The patient (B. C.) had been examined by Mr. Nettleship, who kindly
secured his attendance at South Kensington for the purpose of being
examined by the spectrum and other tests. [Mr. Nettleship states
that this case is without doubt a genuine case of congenital colour
blindness, without any trace whatever of disease.] B. C. is a youth
of 19, who has served as an apprentice at sea. His form vision is
perfect, and he is not night blind. He can see well at all times,
though he states that on a cloudy day his vision seemed to be slightly
more acute than in sunshine. He was first requested to make matches
with the Holmgren wools in the usual manner, with the result that he
was found to possess monochromatic vision. He matched reds, greens,
blues, dark yellows, browns, greys, and purples together; and it was a
matter of chance if he selected any proper match for any of the test
colours. Finally, when pressed, he admitted that the whole of the heap
of wools were “blue” to him, any one only differing from another in
brightness. The brighter colours he called “dirty” or “pale” blue,
terms which eventually proved to be synonymous. We then examined
him with patches of monochromatic spectrum colours by means of the
colour patch apparatus. He designated every colour as “blue,” except a
bright yellow, which he called white, but when the luminosity of this
colour was reduced he pronounced it a good blue. So with white, as the
illumination was decreased, he pronounced it to pass first into dirty
blue, and then into a full blue.
Colour discs were then brought into requisition, and it was hard at
first to know how to make the necessary alterations, owing to the terms
he employed to express the difference which existed between the inner
disc and the outer grey ring. By noting that a pale “blue” passed into
a pure blue when the amount of white in the outer ring was diminished,
and that the inner disc was described as “pale” or “dirty” when the
outer ring was described as “a very full blue,” we were enabled to make
him match accurately a red, a green, and a blue disc separately with
mixtures of black and white.
The following are the equations:--
360 red = 315 black + 45 white.
360 green = 258 black + 102 white.
360 blue = 305 black + 55 white.
With these proportions he emphatically stated that all were good blues,
and that the inner disc and outer ring were identical in brightness and
in colour.
It may be remarked that this is a case of congenital colour blindness,
and that there is reason to believe that some of his ancestors were
colour blind.
Before using the discs an attempt was made to ascertain the luminosity
of the spectrum as it appeared to him. His readings, however, were so
erratic that nothing could be made out from these first observations,
except to fix the place of maximum luminosity, the terms “pale” and
“dirty” puzzling us as to their real meanings. After the experience
with the discs we had a clue as to what he wished to express by pale or
dirty blue, which only meant that the colour or white was too bright,
and on making a second attempt he matched the luminosities of the two
shadows as easily as did P. and Q., the other cases of monochromatic
vision. The method adopted was to diminish the white light illuminating
one shadow to the point at which he pronounced it a good blue, when
a slight alteration in the intensity was always sufficient to secure
to his eye equality of luminosity between it and the coloured shadow
without his perceiving any alteration in the saturation.
[Illustration: FIG. 34.
B. C.’s Luminosity and Extinction Curves.]
The curve of luminosity, Fig. 34, is a very remarkable one, being
different in character to that of P. and Q., the maximum being well on
the D side of E. A great falling off in the luminosity when compared
with that measured by the normal eye will be noticed both in the
blue and in the red. (For measures see page 225.) The evidence was
therefore presumptive that B. C.’s colour sensation was neither red nor
blue, but probably a green.
The next test was made to throw light on this point. He made
observations of the extinction of the different parts of the spectrum.
His observations were very fair, except on the violet side of F, where
they became slightly erratic, but by requesting him to use all parts
of his retina to obtain the last glimpse of light, a very concordant
curve resulted, as shown in Fig. 34. Some of his observations at this
part were evidently made with the centre of the retina, for they gave
readings which, when the “persistency” curve was calculated, and
these observations treated as part of the extinction, agreed with the
luminosity curve. We may, therefore, conclude that B. C. has a region
in the retina in which there is an absorbing medium corresponding to
the yellow spot of the normal eyed. This is diagrammatically shown in
Fig. 34 by the difference in height of ordinates in the persistency
(dotted) and the luminosity curves. On the red side of the maximum the
two curves are practically identical, except from Scale number 54. At
this point it is probable that the white light which illuminated the
prism vitiated the readings to some degree. At the violet end something
similar, doubtless, occurs, but it is masked by the difference that
exists in the extinction by the central part of the retina and that of
the whole eye.
It must, however, be remarked that the amount of reduction of the
intensity of a ray to produce extinction is very different for B. C.
and for the normal eyed, or for the red- and green-blind or for P. and
Q. B. C. can bear nearly 200 times less reduction for the rays near
E. We have already pointed out that the same is practically the case
with M., whom we presume to be violet-blind. We may therefore deduce
the fact that the monochromatic vision in this case is of a totally
different type to that of P. and Q., and that the last sensation to be
lost is the same as that of M. If any violet sensation were present in
either, the fact would be made evident by the order of the extinction.
The sensation of B. C. is thus apparently the green sensation, though
that this particular sensation is exactly the same as that absent in
the green-blind is not certain.
The observations made by the different types of the colour blind
seem to me to throw great light on the theory of colour vision. They
show that when the violet sensation is present, according to the
Young theory, the extinction shows its presence; and that where this
sensation is absent, the reduction of light necessary to produce
extinction is greatly less, and may with great certainty be attributed
to a different sensation being the final one to disappear.