A Standard Light--Formation of the Spectrum by Prisms and by the
Diffraction Grating--Wave-lengths of the principal Fraunhofer
Line--Position of Colours in the Spectrum.
As we have to turn to the spectrum for pure and simple colours, from
which we may produce any compound colour we may wish to deal with, we
will first consider the light with which we shall form it. A spectrum
may be produced from any source of light, such as sunlight, limelight,
the electric light, gaslight, or incandescence electric light, as also
from incandescent vapours, or gases; but it is only a solid which is, or
is rendered incandescent, that will give us a _continuous_ spectrum, as
it is called, that is, a spectrum which is unbroken by gaps of
non-luminosity, or sudden change of brightness, throughout its length.
Fig. 1.--Spectrum of Sunlight.
The great desideratum for the study of colour is a light which not only
gives a practically continuous spectrum, but one which is produced by
the radiation of matter which is black when cold, and which can be kept
at a constantly high temperature. We have purposely said "black" in the
sentence above, since it is believed that differently coloured bodies,
when heated to equal temperatures, might not give the same relative
intensities to the different parts of the spectrum, the variation being
dependent on the colour of the heated body. A black body must always
give the same visible spectrum when heated to the same temperature. The
spectrum of sunlight (Fig. 1) is not continuous, as we find it crossed
by an innumerable number of fine lines of varying breadth and blackness.
This want of continuity would not be fatal to its adoption were it
possible to use it outside the limits of our atmosphere, as then, unless
the temperature of the sun itself changed, the spectrum produced would
be invariable; but unfortunately the relative brightness or luminosity
of the different parts of the spectrum varies from day to day, and hour
to hour, according to the height of the sun above the horizon (see Chap.
VI.); and its integral brightness varies according to the clearness of
the sky. It is evident then, that, as a reference light, sunlight is
most unsuitable, so we may dismiss it from our possible standards.
Fig. 2.--The Carbon Poles of an Electric Light.
By the process of elimination we may arrive at the light upon which we
can rely, for the purpose we have in view, viz. the production of a
spectrum of moderate size, and sufficiently bright to be well viewed
when projected upon a screen. For some purposes, as for instance in
becoming acquainted with the general character of the spectrum, a
feebler light, such as gaslight, or light from electrical glow lamps,
may be employed, since the spectrum may be viewed directly by the eye
without the intervention of a screen. They have two drawbacks for our
object: one being the want of general intensity, and the other the
feeble luminosity of blue and violet rays in their spectrum (see page
110). The limelight we can also dismiss for want of steadiness. Its
whiteness and luminosity varies according to the oxygen playing on the
lime cylinder, rendering the relative intensities of the different parts
of the spectrum so erratic as to make it unreliable. This leaves the
(electric) arc-light as the only one which is really available. Remember
how the arc-light is produced. A current of electricity passes between
the ends of two thick black carbon rods, or poles as they are called,
through an air space of small interval, and the passage of the current
renders the tips of these rods white-hot (Fig. 2). The centre of the end
of one pole, called the positive pole, where a crater-like depression is
formed, is the part which attains the whitest heat, and its temperature
seems to be constant, and to be that of the volatilization of carbon.
Numerous experiments have been made by the writer, and he has found that
the light emitted by this crater in the positive pole is, within the
limits of the error of observation, always of the same whiteness, and
consequently gives a spectrum which is unvarying in the proportionate
intensities of the different colours. When the experiments made to
determine the luminosity of the spectrum are described, the method of
ascertaining this will be readily understood.
In the spectrum produced by this light there are two places in the
violet where there are bands of violet lines slightly brighter than the
general spectrum. They are principally due to the light emitted from the
incandescent vapour of carbon, which is volatilized and plays between
the two poles (see Fig. 2); but as these bands are of but small visual
intensity, and situated towards the limit of the visible spectrum, they
do not interfere with eye-measures of colours, though they do, to a
certain extent, to the analysis of radiation by photography. If we throw
the positive pole a little behind the negative pole we can, however,
considerably mitigate this evil. We can separate the carbon rods to such
a degree that the white-hot crater faces the observer, and a good deal
of the arc is hidden. This is well seen in the figure.
We have now described the light we have adopted, and the reasons for
adopting it; and having obtained our light, we can now consider by what
plan we shall form our spectrum. There are two ways open to us--one by
glass prisms, and the other by a diffraction grating. Glass prisms
separate white light, or indeed any light, into its components, from the
fact that the refraction of each coloured ray differs from every other.
Thus the red rays are least refracted, and the violet the most, and the
yellow, green and blue are intermediate between them, being placed in
the order of least refrangibility. Between these there is of course
every shade of simple colour, one melting into the other. In order to
form a pure and bright spectrum with prisms, in a room of limited
dimensions, we have to use certain auxiliary apparatus which are not
positively essential, though convenient. The real essentials to form a
spectrum are a narrow slit, a glass prism, with perfectly plane faces,
and a lens. If this be the only apparatus available, the slit must be
placed at a long distance from the prism, the beam of light must pass
through the slit on to the prism, and the lens must be placed at such a
distance from the slit that it forms a sharp image on a screen. When the
light passes through the prism, the screen will have to be rotated in
the arc of a circle, so that its distance from the slit measured along
the line of the ray to the prism, and from the prism to the screen, is
the same as it would be without the intervening prism. An apparatus of
this description is not convenient, however, as it requires much more
space than is often available. If a lens be placed between the slit and
the prism, at exactly its focal length from the former, the light
entering the slit will, after passage through the lens, emerge as
parallel rays, that is, they will emerge as they would do if the slit
were placed at an infinite distance from the observer.
The focal length of this collimating lens need not be greater than
twelve to eighteen inches, so that the great space required by the
cruder apparatus is very much curtailed. The lens and slit are mounted
one at each end of a tube of the necessary length, and are thus handy to
use.
Instead of one prism two or three may be used, giving an angular
dispersion of the spectrum two or three times respectively greater than
that which would be given by only one prism; consequently to obtain a
given length of spectrum with the increased dispersion, the focal length
of the lens used to focus the image on the screen may be diminished.
The drawback to the use of prisms is that the dispersion of the red end
of the spectrum is much less than that of the blue end, and is apt to
give a false impression as to the relative luminosities of, and length
of spectrum occupied by, the different colours. In some text-books it is
told us that the diffraction grating gives us a dispersion which is in
exact relation to the wave-length. This is not true, however, as it can
only give one small portion in such relationship, and that only when it
is specially set for the purpose. The subject of diffraction is one into
which it would be foreign to our purpose to wander. We may say that for
measures such as we shall make, it is handier to employ prisms, as the
prismatic spectrum is more intense than the diffraction spectrum. This
can be readily understood when we consider the subject even
superficially. If we throw a beam of light on a grating which contains
perhaps some 14,000 parallel lines in the space of one inch in width,
the lines being ruled on a plane and bright metallic surface, and
receive the reflected beam on a screen, the appearance that is presented
is a white central spot, together with six or seven spectra of gradually
diminishing brightness on each side of it, all except the first pair
overlapping one another. That these different spectra do exist can be
readily shown by placing in the beam a piece of red glass, when
symmetrical pairs of the red part of the spectrum will be found, one of
each pair being on opposite sides of what will now be the central red
spot. Half the light falling on the grating is concentrated in this
central spot, and the remaining half goes to form the spectra; the pair
nearest the central spot being the brightest. We thus are drawn to the
conclusion that at the outside we can only have less than one-quarter of
the incident light to form the brightest spectrum we can use. With two
good prisms we use at last three-fourths of the incident light, so that
for the same length of spectrum we can get at least three times the
average brightness that we should get were we to employ a diffraction
grating.
We must now refresh the reader's memory with a few simple facts about
light, in order that our meaning may be clear when we speak of rays of
different wave-lengths. Every colour in the spectrum has a different
wave-length, and it is owing to this difference in wave-length that we
are able to separate them by refraction, or diffraction, and to isolate
them. Light, or indeed any radiation, is caused by a rhythmic
oscillation of the impalpable medium which we, for want of a better
term, call ether, and the distance between two of these waves which are
in the same phase is called the wave-length of the particular radiation.
The extent of the oscillation is called the amplitude, which when
squared is in effect a measure of the _intensity_ of the radiation. Thus
at sea the distance between the crests of two waves is the wave-length,
and the height from trough to crest the amplitude; and the intensity, or
power of doing work, of two waves of the same wave-lengths but of
different heights, is as the square of their heights. Thus, if the
height of one were one unit, and of the other two units, the latter
could do four times more work than the former. The waves of radiation
which give the sensation of colour in the spectrum vary in length, not
perhaps to the extent that might be imagined, considering the great
difference that is perceived by the eye, but still they are markedly
different. The fact that the spectrum of sunlight is not continuous, but
is broken up by innumerable fine lines, has already been alluded to.
The position of these lines is always the same, as regards the colour in
which they are situated, and is absolutely fixed directly we know their
wave-length; hence if we know the wave-lengths of these lines, we can
refer the colour in which they lie to them. Now some lines of the
solar-spectrum are blacker and consequently more marked than others, and
instead of referring the colours to the finer lines, we can refer them
to the distance they are from one or more of these darker lines, where
these latter are absolutely fixed; in fact they act as mile-stones on a
road.
In the red we have three lines in the solar spectrum, which for sake of
easy reference are called A, B and C; in the orange we have a line
called D, in the green a line called E, in the blue F, in the violet G,
and in the extreme violet H. These lines are our fiducial lines, and all
colours can be referred to them. The following are the wave-lengths of
these lines, on the scale of =1/10,000,000= of a millimetre as a unit
A 7594
B 6867
C 6562
D 5892
E 5269
F 4861
G 4307
H 3968
When the spectrum is produced by prisms the intervals between these
lines are not proportional to the wave-lengths, and consequently if we
measure the distance of a ray in the spectrum from two of these lines,
we have to resort to calculation, or to a graphically drawn curve, to
ascertain its wave-length. For the purpose of experiments in colour the
graphic curve from which the wave-length can immediately be read off is
sufficient. The following diagram (Fig. 3) shows how this can be done.
The names and range of the principal colours which are seen in the
spectrum has been a matter of some controversy. Professor Rood has,
however, made observations which may be accepted as correct with a
moderately bright spectrum. If the spectrum be divided into 1000 parts
between A in the red, and H, the limit of the violet, he makes the
following table of colours.
+---------------+--------------------------------+
| Scale. | Colour. |
+---------------+--------------------------------+
| 0 to 149 | Red. |
| 149 to 194 | Orange red. |
| 194 to 210 | Orange. |
| 210 to 230 | Orange yellow. |
| 230 to 240 | Yellow. |
| 240 to 344 | Yellow green and green yellow. |
| 344 to 447 | Green and blue green. |
| 447 to 495 | Azure blue. |
| 495 to 806 | Blue and blue violet. |
| 806 to 1000 | Violet. |
+---------------+--------------------------------+
Fig. 3.--Curve for converting the Prismatic Spectrum into Wave-lengths.
In the above scale (Fig. 3) A = 0, B = 74·0, C = 112·7, D = 220·3,
E = 363·1, F = 493·2, G = 753·6, H = 1000.
These are the main subdivisions of colour, but it must be recollected
that one melts into the other. When the spectrum is very bright the
colours tend to alter in hue; thus the orange becomes paler, and the
yellow whiter, and the blue paler. On the other hand, if the spectrum be
diminished in brightness the tendency is for the colours to change in
the opposite direction. Thus the yellow almost disappears and becomes of
a green hue, whilst the orange becomes redder, and the spectrum itself
becomes shorter to the eye than before.
Let us strictly guard ourselves, however, from the criticism that all
eyes see not alike. Suffice it to say that the above table is correct
for the ordinary or normal eye, and does not necessarily apply to those
who have defective vision as regards colour sensation.