Not only, however, may we lose a sense of colour, but we may also
lose all sense of light by reducing the energy of the different rays.
We have seen that colour goes unequally from the different parts of
the spectrum. We may therefore prognosticate that the light itself
may disappear more rapidly from some parts than from others. You will
scarcely, however, I think, be prepared for the enormous difference
which exists in the stages of disappearance of the grey of the reduced
red and of that of the reduced green.
[Illustration: FIG. 27.]
But how are we to measure this extinction of light at the different
parts of the spectrum? This is a problem which I have attacked during
the last few years by a variety of methods; but as is the case with
almost every scientific problem, when the mode of attack is reduced to
its simplest form, it yields the more readily to solution. If we have
a box, like that figured in Fig. 27, and combine it with our colour
patch apparatus, the problem is solved. B B is a closed box 3 feet
long and about 1 foot high and wide, having two similar apertures 1½
inch in diameter in the positions shown. The aperture at the side is
covered on the inside by a piece of glass _a_, ground on both sides,
and a tube T is inserted, in which diaphragms, D, of various apertures
can be inserted at pleasure. The most convenient form of diaphragm is
that supplied with photographic lenses--an iris diaphragm. E is a tube
fitted at the end of the box through which the screen S is viewed. S
is black except in the centre, where a white disc is fastened to it. A
mirror, M, placed as shown, reflects the light scattered by the ground
glass on to the screen S. The rotating sectors are placed where shown,
and are in such a position that they can be readily adjusted by the
observer. The patch of any desired colour of the spectrum is thrown
on _a_, and an appropriate size of diaphragm used, so that when the
sectors are not less than 5° to 10° open, the light totally disappears.
We can now make observations throughout the whole spectrum, and knowing
the value of the different apertures of the diaphragm and the angular
opening of the rotating sectors, we can at once find the amount of
reduction of the particular part of the spectrum that is being required
in order to just extinguish all traces of light from the white disc
at the end of the box. From these measures we can readily construct
a curve or curves which will graphically show the reduction given
to the different parts of the spectrum. Fig. 28 gives the curve of
extinction for ordinary normal colour vision. The spectrum was of such
a brilliance that the intensity of the square patch of light formed on
_a_ of the orange light (D) was exactly that of an amyl-acetate lamp,
placed at one foot distance from the receiving screen. Knowing this,
the actual luminosity of all the other rays of the spectrum can be
derived from the curve of luminosity (see Fig. 20). Extinguishing the
various parts of the spectrum by this plan, it is found that the red
rays cease to stimulate the retina sufficiently to give any appearance
of light long before the green rays are extinguished. It is only the
rays in the extreme violet of the spectrum, and which consequently
possess very feeble luminosity, that make any approach towards
requiring the same amount of reduction as the red rays.
[Illustration: FIG. 28.]
There is the fact to remember in making these measures in the extreme
red and the extreme violet, that the luminosities of the colours are
so small that the illumination of the prism itself, by the white
light falling on it, has to be taken into account, since it forms
an appreciable portion of the patch of feeble colour. By placing a
proper shade of blue or red glass in the front of the collimator
slit this white light disappears or becomes negligible, and when the
absorption of the coloured glass is known from measurement, we can
get a very accurate measure of the extinction of these parts. Some
people may propound the idea that the rotating sectors may in such
kind of measurements give a false result. Now such a criticism is
quite fair, and it is absolutely necessary that it should be answered.
Well, to test the accuracy or the reverse of the assumption that such
measures are correct, the following small piece of simple apparatus
was devised. A and B (Fig. 29) are two mirrors placed at angles of 45°
to the angle of incidence of the beam. The path the beam takes can be
readily ascertained from the figure. This piece of apparatus was placed
in position in front of the spectrum, and the reflected beams used
to form the patches of colour. For convenience only a small pencil of
light was allowed to issue from the prism, a diaphragm of some ½-inch
in diameter being placed in front of it. This allows a spot of any
desired colour to fall on the screen, the ground glass being removed.
The slit through which the spectrum colours pass is moved along the
spectrum, and a position is arrived at where the last glimmer of light
disappears.
[Illustration: FIG. 29.]
The mirrors A and B may both be of plain glass blackened with smoke on
one side, or one may be plain glass and one silvered, or they both may
be silvered. This, with the power possessed of altering the aperture
of the slit of collimator, puts us in possession of ample means of
making our measures. We may also use the ground-glass arrangement and
use different diaphragms, which puts a further power of variation in
our hands. I may at once state that the resulting measurements fell on
the curves, obtained by measurements made with the rotating sectors, a
sufficient proof that the sectors may be used with confidence. There is
still another method which avoids a resort to the sectors. A tapering
wedge of black glass can be moved in front of the colour slit, and a
different thickness of glass will be required to cause the extinction
of each colour. Recently I have modified the extinction box, more
particularly for the purpose of using it where the spectrum is to be
formed of a feeble light, such as that of an incandescent lamp or a
candle. If a really black wedge could be obtained, this would seem to
be the best method, but no glass is really black. We have, therefore,
to make a preliminary study of the wedge to ascertain accurately the
absorption co-efficients for the different rays, a piece of work which
requires a good deal of patience, but which, when done, is always at
command.
In Fig. 28 two branches of the curves are given at the blue end of the
spectrum; one is shown as the extinction for the centre of the eye,
and the other of the whole eye. Of course the former observations were
made by looking direct at the spot. This may appear a very easy matter,
but it is not really so simple as it sounds. It is curious how little
control there is over the absolute direction of the eyes when the
light has almost disappeared. The axes of the eyes are often directed
to quite a different point. When the extinction for the whole eye is
made, the readings are really much easier, as then the eye roams where
it likes, and a final disappearance is noted. When the eye has once
been invested with a roving commission, it is hard to control it. In
making these observations it was therefore advisable to have data for
the first branch of the curve, before commencing to observe for the
later. The main cause of difference between the two branches of the
curve is due to the absorption by the yellow spot.
It might be thought that with the curves (Fig. 28) before us, we have
learnt all we can regarding the extinction of light, but is it so?
Surely we ought to know something as to the reduction necessary for
extinction of the different parts of the spectrum when they are all of
equal luminosities and of ordinary brightness.
We arrive at this by simple calculation. Supposing we have two
luminosities, _one double the other_, it does not require much thought
to find out that you have to reduce the greater luminosity twice as
much as the other in order for it to be just extinguished. In other
words, if we multiply the extinction by the luminosity, we get what we
want. Now, in the curves before us, we have taken the luminosity of the
yellow light near D as one amyl-acetate lamp, and that has a height
in the curve showing the spectrum luminosity very closely approaching
100. We may, therefore, multiply the extinctions of a ray by the value
of its ordinate in the luminosity curve and divide the result by 100,
and this will give us the extinction of each colour, supposing it had
the luminosity of an amyl-acetate lamp. A portion of the curve so
calculated is shown in the same diagram (Fig. 28) as a dotted line. It
appears at the violet end as an approximately horizontal line, and then
starts rapidly upwards, and would, if carried on to the same scale,
reach far out of the diagram; but at the extreme red it would be found
to bend and again become horizontal. I would have you notice that the
same is true not only for the extinction observed with the centre of
the eye through the yellow spot, but also for the whole eye. Such
straight, horizontal parts of the curve must mean something.
[Illustration: FIG. 30.]
In the diagram (Fig. 16) of colour sensations we see that in each
of these two regions there is but one sensation excited, viz. the
violet and the red. Now, if these sensation curves mean anything, the
reduction necessary to produce the extinction of the same sensation
when equally stimulated should prove to be the same, for there is
no reason to the contrary, but exactly the reverse. _Primâ facie_,
then, taking the Young theory as correct, we may suppose that these
horizontal parts are due to the extinction of one sensation. Let us
treat it as such, and go back to the original extinction curve shown
in the continuous lines. The parts of the curve which lie over the
fairly horizontal dotted line, at all events, should be the extinction
curve of the same sensation, but more or less stimulated or excited.
As before explained, if we have double the stimulation at one part of
the spectrum to that we have at another, the reduction of the greater
luminosity to give extinction will be double that of the lesser. If,
then, we take the _reciprocals of the extinction_, it ought to give
us a curve which is of the form of some colour sensation; and when we
arrive at the maximum, we may for convenience make that ordinate 100,
and reduce the other ordinates proportionally. This has been done in
Fig. 30 in the curves C and D. For the sake of a name my colleague and
myself have named such curves “persistency curves.” Perhaps some other
name might be more fitting; but still a poor name is better than none
at all.
When the persistency curve was scrutinized to see what might be
taken as its full signification, I must confess that the result
astonished us somewhat, though we ought not to have been surprised.
The persistency curve C, when applied (in a Euclidean sense) to the
curve of luminosity recorded for the men who had monochromatic vision,
almost exactly coincided with it. In other words, by far the largest
part of the extinction was due to the extinction of the sensation
which in the monochromatic vision was alone excited. If this be not
the case, there is something in colour vision which no theory which
I am acquainted with can account for. Then, again, the persistency
curve agrees with the curve of luminosity when the intensity of the
spectrum is very feeble, which is another coincidence of a remarkable
character which some theory should explain. [Fig. 30 gives, besides
the persistency curves, the luminosity curves of the normal eye, of
monochromatic vision, and of the violet-blind; and an exaggerated curve
of the difference between the normal luminosity curve and that of the
violet-blind, and others which I think will be found useful for general
reference.]
What sensation is it that is last extinguished, and which is possessed
by a certain class of colour vision? In the Young theory it can only
be the violet sensation. It is certainly not the green, and much less
the red. It does not correspond, however, very well with the violet
sensation shown in Fig. 16, but more with one which should be in the
blue.
In making the extinctions of light, it is quite necessary that certain
precautions should be taken to avoid error. All my audience know that
when going from bright daylight into a cellar, in which only a glimmer
of light is admitted, but little can be seen at first, but that, as the
eye “gets accustomed” to the darkness, the surroundings will begin to
be seen, and after several minutes what before was blackness comes to
be invested with form and detail. So it is with the extinction of light
in the apparatus described. Observations carried on before the full
sensibility of the eye is attained are of no value. A recorded set of
observations will show this. A light of a certain character was thrown
on the extinction box, to be extinguished, and the observer entered
the darkened room from the full glare of daylight. The eye was placed
at the eye end and kept there, and the extinctions were made one after
the other till they became very fairly constant. The following is the
result:--
Times of Observation. Readings.
At the commencement 1·0
After 38 sec. 3·2
After 53 sec. 4·9
After 1 min. 11 sec. 6·9
After 1 min. 44 sec. 10·5
After 2 min. 43 sec. 17·0
After 3 min. 44 sec. 27·5
After 4 min. 52 sec. 43·0
After 5 min. 59 sec. 63·0
After 6 min. 41 sec. 78·0
After 7 min. 28 sec. 89·0
After 8 min. 32 sec. 96·0
After 10 min. 46 sec. 103·0
After 12 min. 103·0
(For convenience the first reading is unity; the other numbers are
the _inverse_ of the extinction value.)
The eye apparently, under the conditions in which these observations
were made, was at least 100 times more sensitive to very faint light
after twelve minutes than it was at the beginning, and that then
concordant readings could be made. It will now be quite understood that
before any serious measures can be made this interval must elapse,
and also that the light, finding its way to the end of the box to
illuminate the spot, should never be strong, otherwise the eye might
lose its sensitiveness.