Before quitting the measurement of luminosity, it may be as well to
see whether the curves described are the same whatever the brilliancy
of the spectrum may be. We can easily experiment with a very reduced
brightness. Upon the screen we have an ordinarily bright spectrum. As
the slit, through which the white light forming the spectrum comes, is
narrowed, there is an evident change in the relative brightness of the
different parts, though the energy of every ray must be proportionally
reduced. The red is much more enfeebled than the green, and in
brightness the green part of the spectrum looks much more intense
than the yellow, which is ordinarily the brightest part. This we have
assured ourselves of not only by casual observation, but also by direct
measurement. Perhaps I can make this even more decisive to you. Two
slits are now in the ordinarily bright spectrum, one in the red, and
the other in a green which is near the E line of the solar spectrum.
Instead of using one lens to form a single colour patch of mixed light,
two parts of a lens, appropriately cut and of the same focal length as
the large combining lens, are placed in front of the slits, one bit of
lens before each. This artifice enables us to throw the patch of red
on one white surface, and the patch of green light on another adjacent
to it. By opening or closing one or other of the slits the brightness
of the two patches of light are so arranged that there is no manner
of doubt but that the red is the brighter of the two. The absolute
energies of the rays forming each of the patches are proportionally
reduced by closing the slit of the collimator, as before. At one stage
both patches appear of about the same intensity. This might be taken
for an error in judgment, but to make the change that takes place
perfectly plain to you, the rotating sectors are introduced in front of
the two slits, and the rays now pass through them. The apertures of the
sectors are gradually closed, and we now come to such a reduction that
the red is absolutely invisible; but the green still shines out. It is
losing its colour somewhat, and appears of a bluish tint. The reason of
this change of hue in the latter we shall shortly see. The sectors are
withdrawn and the red re-appears, and is as bright as the green. The
slit of the collimator is next opened, and there is no doubt that the
red is much brighter than the green, as it was purposely made at the
beginning of the experiment. The same class of experiment might have
been repeated with the green and violet or the red and violet, and the
same kind of results would have been obtained. The violet would have
been the last to disappear when the green was so reduced in luminosity
that it appeared in the ordinary brilliant spectrum to be equal to
the violet ray selected. When the green was of the luminosity given
by a slit equal in width to that of the violet, the violet would have
disappeared first, owing to its feeble brightness to begin with. Now,
if we measure a feebly illuminated spectrum we must adopt some special
means to exclude all light, except that of the comparison light and the
ray to be measured. This we can do by the box which is shown in the
next diagram (Fig. 24).
[Illustration: FIG. 24.]
At one end of a box, shown in plan, is an eye-piece, E. The other
end has at its centre a white square of paper of 1½-inch scale. The
monochromatic beam _a_, coming from the spectrum through the slit S
and the reference beam _b_ of white light, are reflected from glass
mirrors M₁, M₂ to apertures in opposite sides of the box, and from
close to these apertures by the right-angled prisms P₁ P₂, so as to
fall on and cover S. Rods R₁, R₂ are inserted in the box in the
paths of the beams, so that the opposite halos of S are illuminated.
Diaphragms inside the box cut off any stray light, and rotating sectors
placed at A and B regulate the intensity of the beams as required.
The sector A is rotated with a previously determined-on aperture;
the white light coming through B is altered till the luminosity of
the two on the screen, as seen through E, are the same. Every part
of the spectrum can be measured in this way; the result is shown in
the diagram. Fig. 25 (the measures will be found at page 215 in the
appendix). In this case the orange light at D where it fell on the
screen was equal to 1/132 of an amyl-acetate light, which, in its turn,
is closely ·8 of a standard candle. In the same figure the luminosity
curve of the ordinary bright spectrum is given for reference, and it
can be seen how the point of maximum luminosity is shifted into the
green, lying almost over the E line of the solar spectrum. The maximum,
of course, has been made 100 as before, for had it been drawn to the
same scale as the other, the form of the curve would not have been
demonstrated. There is a remarkable resemblance between it and the
curve of luminosity of the monochromatic vision, and such a resemblance
can scarcely be fortuitous. As a matter of fact, in this we seem to
have come to the final curve for low luminosities, and is almost the
same as that observed when the spectrum is reduced to such an extent
that it is colourless throughout, a condition that it can assume, as we
shall see very shortly. When the spectrum is rather more luminous, it
gives a curve of luminosity which is similar to that of the ordinary
spectrum when measured by a red-blind person. Here, then, we have an
indication that a person with normal vision passes through a stage of
red-blindness, as the intensity is diminished before he arrives at
absolutely monochromatic vision.
[Illustration: FIG. 25.]
This investigation is of practical as well as theoretical interest, as
General Festing and myself quickly discovered when we first made it.
The curious colour of a moonlight landscape is entirely accounted for
by it. White light becomes greenish-blue as it diminishes in intensity,
and the reds and yellows, being reduced or absent, are not reflected by
surrounding objects. Hence, moonlight is cold, whilst the sunlight is
warm owing to their presence.
When measuring these low luminosities, the various colours will in a
great measure disappear. Part of the spectrum will be of that peculiar
grey which was shown you in the experiment with the incandescent
light (p. 34). By further experiment it is possible to arrive at an
approximate determination of the point where all colour vanishes from
the different parts of the spectrum. We use the same apparatus (Fig.
24) as before, the only difference being that each of the sectors
is movable during rotation. The apertures of those through which
the colour passes are reduced till all colour on the screen just
disappears, the point being arrived at by a comparison with the white,
which is itself also reduced. The apertures of the first sector alone
need be noted, and from these readings the diagram (Fig. 26) is made
(for measures, see page 216).
[Illustration: FIG. 26.]
This extinction of colour is one which often occurs, but is seldom
noticed. The figure tells us that the orange is about the last colour
of the spectrum left, some of the others still appearing as greys. The
next to retain its colour is the green, and the most rapid to lose them
are the red and violet. It must not be supposed that the colours remain
of the same hue up to the time that they vanish. Pure spectrum red (red
sensation) remains the same up to the last, but the scarlet becomes
orange, and the orange yellower, and the green bluer. This is what
would be predicted from the Young theory if the order of extinction of
sensation be red, green, violet. This we shall see is the case. At
nightfall in the summer the order of disappearance of colour may often
be seen; orange flowers may be plainly visible, yet a red geranium
may appear black as night; the green grass will be grey when the
colour of the yellow flowers may yet be just visible. An early morning
start in the autumn before daybreak will give an ample opportunity of
satisfying oneself as to the order in which colours gradually re-appear
as daybreak approaches. Red flowers will be at the outset black,
whilst other colours will be visible as grey. As more light comes from
the sky the pale yellow and blue flowers will next be distinguished,
though the grass may still be a nondescript grey. Then, as the light
still increases, every colour will burst out, if not in their full
brilliance, yet into their own undoubted hue.