Extinction of Colour by White Light--Extinction of White Light by
Colour.
In the last chapter we have shown the impossibility of matching the hue
of the simple colours between the violet and the green, unless a certain
and appreciable quantity of white light be added to them. We will now
turn to a phase of colour measurement which will materially help us to
see why, in some cases, the addition of white light to the simple
spectrum colours, between the red and green, does not appear necessary
in order to make a match with a mixture of red and green.
We will ask ourselves two questions: one is, whether any colour, and if
so how much, can be added to white without appearing to the eye? and the
other, if any, and if so how much, white light can be added to a colour
without its being perceived?
Perhaps one of the readiest methods of explaining exactly what we mean
is by a rotating disc. Suppose we have a red disc, of nine or ten inches
in diameter, and at every one inch from the centre paste on it a white
wafer about one-eighth of an inch in diameter, and cause it to rapidly
rotate. On examination we shall find that pink rings will be formed by
the combination of the white and red near the centre, but that towards
the margins no rings will be visible, owing of course to more red being
combined with the same amount of white. This shows that the eye is only
sensitive to a certain degree, and cannot distinguish a very small
diminution in colour purity. The intensity of the light has something to
do with the number of these pink rings which are visible, as may readily
be tested in a room. If the rotating disc be placed near a window, and
the number of rings visible be counted, a different number will be
visible when it is placed in a dark corner. A kindred experiment is to
place red circular wafers upon a white disc, and note the rings visible.
This gives the sensitiveness of the eye for the diminution in intensity
at the other end of the scale. It will be found that there is a marked
difference between the two.
Fig. 32.--Diaphragm in front of Prism.
It is more instructive if we experiment with pure colours, and so we
must resort to our colour patch apparatus described in Fig. 6. If a
small circular aperture about quarter of an inch in diameter be cut in a
card, and placed in front of the prism nearest the camera lens (Fig.
32), the colour patch, instead of being an image of the face of the
prism, will be an image of the circular hole, and when the slit is
passed through the spectrum we shall have a coloured spot on the screen,
on which we can superpose a patch of white light from the reflected
beam. There are two ways in which we can reduce the intensity of the
spot, by narrowing the slit through which the spectral ray passes or by
placing the rotating sectors in front of the coloured beam. This last,
perhaps, is the readiest plan, as it only involves the reading of the
sector. We can then diminish the intensity of the coloured spot to such
a degree that by its dilution with white light it will entirely
disappear. It will be found that red disappears at a different aperture
of sector to that required for the green, and the green to that for the
blue.
From our previous experiments in chapter VII. we know the luminosity of
the spectrum to the eye, and it will be of interest to see what relation
the luminosity at which the spots of different colour disappear, when
they are so diluted with white light, bear to the total luminosity of
these rays.
In a set of measurements made it was found that the reduced angular
apertures required for the colours indicated by the following were:
B required 300°* of aperture.
C " 56° "
D " 14° "
E " 22° "
F " 150° "
G " 2100°* "
The large numbers marked with an asterisk were obtained by placing the
rotating sectors in front of the white reflected beam.
The light of D had to be reduced to 14° before it was extinguished;
therefore to extinguish the original light of this colour in the
spectrum would require 180/14, or 12·9 times the intensity of the white
light of the reflected beam. With the E light it would take 180/22, or
8·2 times the white light to extinguish it, and so on. If we tabulate
the results in this manner, and take the white light necessary to
extinguish the D light empirically as 98·5, which is its percentage
luminosity in the spectrum of the electric light, we can then compare
the extinguishing factor with the luminosity in each case.
+------------+-------------------------------------------+
| | | White required| |
| |White required| to extinguish | Luminosity |
| Colour. | to Extinguish| the Spectrum, | of |
| | the Spectrum.|with 50 as That| Spectrum. |
| | | required at E.| |
|------------+--------------+---------------+------------+
|near line B | ·6 | 3·9 | 4·9 |
| C | 3·2 | 19·5 | 20·6 |
| D | 12·9 | 78 | 98·5 |
| E | 8·2 | 50 | 50 |
| F | 1·2 | 7·5 | 7·5 |
| G | ·087 | ·56 | ·6 |
+--------------------------------------------------------+
The very close resemblance between the last two columns indicates that
the same luminosity of white light is necessary to extinguish the same
luminosity of most colours, within the limits of observation that is to
say. Indeed the method of extinction was a plan which Draper and
Vierordt essayed, but the results, tabulated from experiments made by
them with the apparatus they employed, give a curve of intensity very
unlike that given in Chapter VII. In these experiments the luminosity of
the orange light corresponding to the D line coming through the slit was
measured, and it was found to be 37·5/180 of the white light. Now
according to the last table but one 14/180 of this light was
extinguished by the full white light, consequently 37·5/180 x 14/180, or
1/62 of the orange light was extinguished by the white light. In other
words, if white light be sixty-two times brighter than the orange
light, the colour of the latter when the two are mixed will be
invisible. The extinction of all colours requires somewhat more light
than this, and a calculation shows that the extinction of every colour
is effected by white light, which is seventy-five times brighter than
the colour. Artists are well aware that a pale wash of a pigment may be
washed over drawing paper, and when dry is invisible to the eye. The
above experiments fully account for it.
The other experiment which was to be tried was to see how much white
light could be extinguished by a colour. There are several ways by which
this can be effected. For instance we may superpose a white dot on the
colour patch by placing a card, in which a circular hole is cut, in the
reflected beam near the prism, from which the reflection takes place; or
by putting a black circular disc of small dimensions pasted on a glass
in the same position, by which means the white light is superposed over
the whole of the colour patch, with the exception of what, when the
colour is cut off, is a black spot; or again by placing a rod to shade
half the patch from the white light, but leaving the whole of it exposed
to the coloured beam. All these methods have been tried, and it appears
that the size of the piece of the patch over which the white light is
thrown may have some effect on the resulting curve, but of one thing
there is evidence, viz. that a great deal more white light can be mixed
unperceived with orange light, than can be with the green, blue, or
violet. From one experiment it was found that 1/36 part of white light
of the same luminosity as the orange could be mixed with the orange and
not be perceived; but that with the green light at E 1/90 would just be
visible, whilst at F in the blue-green the 1/120 could be distinguished.
Looking at these results, and applying them in elucidating the
experiments in which it was attempted, but without success, to match the
intermediate colours between violet and green (of which the light at F
is a case in point), by mixing them together, unless white light were
added to the simple colour; and the success of the other experiment, in
which orange light could be obtained of the same hue as that at D by a
mixture of the red and green, it will be noticed that 3·3 times more
white light can be added to the orange than to the green light at F,
without its perception. The white light produced by the mixture in the
first case might well show when mixed with the green, but might pass
wholly unperceived when mixed with the orange.