Description of Colour Patch Apparatus--Rotating Sectors--Method of
making a Scale for the Spectrum.
Before proceeding further we must describe somewhat in detail two or
three pieces of apparatus to be used in the experiments we shall make.
The first piece was devised by the writer a few years ago, and has got
rid of several objections which existed in older pieces of apparatus. It
is not only useful for lecture purposes, but also for careful laboratory
work. The ordinary lecture apparatus for throwing a spectrum on the
screen is of too crude a form to be effective for the purpose we have in
view; the purity of the colours seen on the screen is more than
doubtful, and this alone unfits it for our experiments. If we want to
form a pure spectrum we must have a narrow slit, prisms with true, flat
surfaces, and lenses of proper curvature. As a rule the ordinary
lecture apparatus for forming the spectrum lacks all of these
requisites.
Fig. 6.--Colour Patch Apparatus.
The accompanying diagram (Fig. 6) will give an idea of the apparatus we
shall employ. On the usual slit S1 of a collimator C is thrown, by means
of a condensing lens L1, a beam of light, which emanates from the
intensely white-hot carbon positive pole of the electric light. The
focus is so adjusted that an image of the crater is formed on the slit.
The collimating lens L2 is filled by this beam, and the rays issue
parallel to one another and fall on the prisms P1 and P2, which
disperse them. The dispersed beam falls on a corrected photographic
lens L3, attached to a camera in the ordinary way. It is of slightly
larger diameter than the height of the prisms, and a spectrum is
formed on the focusing-screen D, which is slewed at a slight angle with
the perpendicular to the axis of the lens L3. This is necessary, because
the focus of the least refrangible or red rays is longer than that of
the more refrangible or blue rays. By slewing the focusing-screen as
shown, a very good general focus for every ray may be obtained. When
the focusing-screen is removed, the rays form a confused patch of
parti-coloured light on a white screen F, placed some four feet off the
camera. The rays, however, can be collected by a lens L4, of about two
feet focus, placed near the position of the focusing-screen, and
slightly askew. This forms an image on the screen of the near surface of
the last prism P2; and if correctly adjusted, the rectangular patch of
light should be pure and without any fringes of colour. The card D
slides into the grooves which ordinarily take the dark slide. In it
will be seen a slit S2, the utility of which will be explained later on.
We shall usually require a second patch of white light, with which to
compare the first patch. Now, although the light from the positive pole
of the carbons is uniform in quality, it sometimes varies in quantity,
as it is difficult to keep its image always in exactly the centre of the
slit. If we can take one part of the light coming through the slit to
form the spectrum, and another part to form the second patch of white
light, then the brightness of the two will vary together. At first sight
this might appear difficult to attain; but advantage is taken of the
fact that from the first surface of the first prism P1 a certain amount
of light is reflected. Placing a lens L5, and a mirror G, in the path of
this reflected beam, another square patch of light can be thrown on the
same screen as that on which the first is thrown, and this second patch
may be made of the same size as the first patch, if the lens L5 be of
suitable focus, and it can be superposed over the first patch if
required; or, as is useful in some cases, the two patches may be placed
side by side, just touching each other.
We are thus able to secure two square white patches upon the screen F,
one from the re-combination of the spectrum, and one from the reflected
beam. If a rod be placed in the path of these two beams when they are
superposed, each beam will throw a shadow of the rod upon the screen.
The shadow cast by the integrated spectrum will be illuminated by the
reflected beam, and the shadow cast by the latter will be illuminated by
the former. In fact we have an ordinary Rumford photometer, and the two
shadows may be caused to touch one another by moving the rod towards or
from the screen. When the illumination of the two shadows by the white
light is equal, the whole should appear as _one_ unbroken gray patch. To
prevent confusion to the eye a black mask is placed on the screen F with
a square aperture cut out of it, on which the two shadows are caused to
fall. If it be desired to diminish the brightness of either patch, it
can be accomplished by the introduction of rotating sectors M, which can
be opened and closed at pleasure during rotation, in the path of one or
other of the beams.
Fig. 7.--Rotating Sectors.
The annexed figure (Fig. 7) is a bird's-eye view of the instrument. A A
are two sectors, one of which is capable of closing the open aperture by
means of a lever arrangement C, which moves a sleeve in which is fixed a
pin working in a screw groove, which allows the aperture in the sectors
to be opened and closed at pleasure during their revolution; D is an
electro-motor causing the sectors to rotate. To show its efficiency, if
two strips of paper, one coated with lamp-black and the other white, are
placed side by side on the screen, and if one shadow from the rod falls
on the white strip, and the other shadow on the black strip of paper,
and the rotating sectors are interposed in the path of the light
illuminating the shadow cast on the white strip, the aperture of the
sectors can be closed till the white paper appears absolutely blacker
than the black paper. White thus becomes darker than lamp-black, owing
to the want of illumination. This is an interesting experiment, and we
shall see its bearings as we proceed, as it indicates that even
lamp-black reflects a certain amount of white or other light.
Having thus explained the main part of the apparatus with which we shall
work, we can go on and show how monochromatic light of any degree of
purity can be produced on the screen. If the slit in the cardboard slide
D be passed through the spectrum when it has been focused on the
focusing-screen, only one small strip of practically monochromatic light
will reach the screen, and instead of the white patch on the screen we
shall have a succession of coloured patches, the colour varying
according to the position the slit occupies in the spectrum. It should
be noted that the purity of the colour depends on two things--the
narrowness of the slit S1 of the collimator, and of the slit S2 in the
card. If two slits be cut in the card D, we shall have two coloured
patches overlapping one another, and if the reflected beam falls on the
same space we shall have a mixture of coloured light with white light,
and either the coloured light or the white light can be reduced in
brightness by the introduction of the rotating sectors. If the rod be
introduced in the path of the rays we shall have two shadows cast, one
illuminated with coloured light, monochromatic or compound, and the
other with white light, and these can be placed side by side, and
surrounded by the black mask as before described.
Fig. 8.--Spectrum of Sodium Lithium and Carbon.
There is one other part of the apparatus which may be mentioned, and
that is the indicator, which tells us what part of the spectrum is
passing through the slit. Just outside the camera, and in a line with
the focusing-screen, is a clip carrying a vertical needle. A small beam
of light passes outside the prism P1; this is caught by a mirror
attached to the side of the apparatus, and is reflected so as to cast a
shadow of the needle on to the back of the card D, on which a carefully
divided scale of twentieths of an inch is drawn. To fix the position of
the slit the poles of the electric light are brushed over with a
solution of the carbonates of sodium and lithium in hydrochloric acid,
and the image of the arc is thrown on the slit. This gets rid of the
continuous spectrum, and only the bright lines due to the incandescent
vapours appear on the focusing-screen (Fig. 8). Amongst other lines we
have the red and blue lines due to the vapour of lithium; the orange,
yellow (D), and green lines of sodium, together with the violet lines of
calcium (these last due to the impurities of the carbons forming the
poles). These lines are caused successively to fall on the centre of the
slit by moving the card D, which for the nonce is covered with a piece
of ground glass, and the position of the shadow of the needle-point on
the scale is registered for each. A further check can be made by taking
a photograph of these lines, or of the solar spectrum, and having fixed
accurately on the scale any one of these lines already named, the
position of the others on the scale may be ascertained by measurement
from the photograph. Now the wave-lengths of these bright lines have
been most accurately ascertained, in fact as accurately as the dark
lines in the solar spectrum. Thus the scale on the card is a means of
localizing the colour passing through the slit or slits. Should more
than one slit be used in the spectrum the positions of each can be
determined in exactly the same way. The most tedious part of the whole
experimental arrangement with this apparatus is what may be called the
scaling of the spectrum.
A fairly large spectrum may be formed upon the screen without altering
any arrangement of the apparatus, when it has been adjusted to form
colour patches. If a lens L6 (see Fig. 6) of short focus be placed in
front of L4 (the big combining lens), an enlarged spectrum will be
thrown upon the screen F, and if slits be placed in the spectrum the
images of their apertures are formed by the respective coloured rays
passing through them, so that the colours which are combined in the
patch can be immediately seen.