Mapping a Spectrum

We shall suppose the spectroscope has more than one prism. Turn the telescope to view some distant object through an open window, and focus it. In doing this adjust first the eye-piece until the cross-wires are seen distinctly, then move the eye-piece and cross-wires by means of the screw until the distant object is clear. The instrument should be focussed so that on moving the eye about in front of the eye-lens no displacement of the image relatively to the cross-wires can be seen.

Remove the prisms, and if possible turn the telescope to look directly into the collimator. Illuminate the slit and focus the collimator until the slit is seen distinctly. Replace one prism and turn the telescope so as to receive the refracted beam. Turn the prism round an axis parallel to its edge until the deviation of some fixed line is a minimum (see §62, p. 311).

For this adjustment we can use a Bunsen burner with a sodium flame. If the prism have levelling screws, adjust these until the prism is level.

To test when this is the case fix a hair across the slit, adjusting it so that when viewed directly it may coincide with the horizontal cross-wire of the eye-piece. The hair will be seen in the refracted image cutting the spectrum horizontally. Adjust the levelling screws of the prism until this line of section coincides with the cross-wire.

In some instruments the prisms have no adjusting screws, but their bases are ground by the maker so as to be at right angles to the edge.

Having placed the first prism in position, secure it there with a clamp, and proceed to adjust the second and other prisms in the same way.

The table of the spectroscope is graduated into degrees and minutes, or in some instruments there is a third tube carrying at one end a scale and at the other a lens whose focal length is the length of the tube. The scale is illuminated from behind by a lamp and is placed so that the rays which issue from the lens fall on the face of the prism nearest the observing telescope, and being there reflected form an image of the scale in the focus of the telescope.

Bring the vertical cross-wire, using the clamp and tangent-screw, over the image of the slit illuminated by the yellow sodium flame and read the scale and vernier, or note the reading of the reflected scale with which it coincides.

Replace the sodium flame by some other source of light the spectrum of which is a line or series of lines, as, for example, a flame coloured by a salt of strontium, lithium, or barium, and take in each case the readings of the reflected scale or of the vernier when the cross-wire coincides with the bright lines.

Now the wave-lengths of these lines are known; we can therefore lay down on a piece of logarithm paper a series of points, the ordinates of which shall represent wave-lengths, while the abscissae represent the graduations of the circle or scale.

If we make a sufficient number of observations, say from ten to fifteen, we can draw a curve through them, and by the aid of this curve can determine the wave-length of any unknown line; for we have merely to observe the reading of the circle or scale when the cross-wire is over this line and draw the ordinate of the curve corresponding to the reading observed. This ordinate gives the wave-length required.⁽¹⁾

A diagram showing the positions of the lines of a spectrum as referred to the circle readings or the graduations of the reflected scale is called a map of the spectrum.

In using the map at any future time we must adjust the scale or circle so that its zero occupies the same position with reference to the spectrum. This can be done by arranging that some well-known line - e.g. D - should always coincide with the same scale division or circle reading.

Instead of using the light from a Bunsen burner with metallic salts in the flame, we may employ the electric spark from an induction coil either in a vacuum tube or between metallic points in air.

If the vacuum tube be used, two thin wires from the secondary of the coil are connected to the poles of the tube - pieces of platinum wire sealed into the glass. The primary wire of the coil is connected with a battery of two or three Grove cells, and on making contact with the commutator the spark passes through the tube. This is placed with its narrow portion close up to and parallel to the slit, and the spectroscope observations made as before. If the spark be taken between two metallic poles in air, the two poles placed in the spark-holder are connected with the secondary and placed at a distance of two or three millimetres apart, and the spark passed between them.

The spark-holder is placed in front of the slit, and either the spark is viewed directly or a real image of it is formed on the slit by means of a convex lens of short focus.

With this arrangement, in addition to the spectrum of the metal formed by the light from the glowing particles of metal, which are carried across between the poles by the - spark, we get the spectrum of the air which is rendered incandescent by the passage of the spark. The lines will probably be all somewhat faint, owing to the small quantity of electricity which passes at each discharge.

To remedy this, connect the poles of the secondary coil with the outside and inside coatings of a Leyden jar, as is shown in fig. 35. Some of the electricity of the secondary coil is used to charge the jar; the difference of potential between the metallic poles rises less rapidly, so that discharges take place less frequently than without the jar; but when the spark does pass, the whole charge of the jar passes with it, and it is consequently much more brilliant Even with the jar, the sparks pass so rapidly that the impression on the eye is continuous.⁽²⁾

In experiments in which the electric spark is used, it is well to connect the spectroscope to earth by means of a wire from it to the nearest gas-pipe; this helps to prevent shocks being received by the observer.

Sometimes after the spark has been passing for some time it suddenly stops. This is often due to the hammer of the induction coil sticking, and a jerk is sufficient to start it again; or in other cases it is well to turn the commutator of the coil and allow the spark to pass in the other direction.

It may of course happen that the screws regulating the hammer of the coil require adjustment

Experiments.

Draw a curve of wave-lengths for the given spectroscope, determining the position of ten to fifteen points on it, and by means of it calculate the wave-length of the principal lines of the spectrum of the given metal.

Map the spectrum of the spark passing through the given tubes.

(1)	See Glazebrook Physical Optics, p. 113.
(2)	The intensity of the spark may often be sufficiently increased without the use of the jar by having a second small break in the cuit between A and C across which a spark passes.