COMPARISON OF SPECTRA.
When once a spectrum has been cali brated, it can be used for calibrating others obtained from the same prism if some method be adopted for placing the same side by side without any relative displacement. This is very conveniently done by covering one-half of the slit with a total reflection prism. To get, for ex ample, the spectra of hydrogen and oxygen in view at the same time for comparison, the tubes are placed at equivalent dis tances from the slit, as shown by Fig. 767.
The rays from the hydrogen 'iss directly into the uncovered top half of the slit, and are brought to a focus in the bottom half of the telescope tube. The side b c of the prism reflects the lower half of the hydrogen rays so that they do not enter. The oxygen rays pass at right angles to the surface through the side a b of the prism, and are reflected internally at the surface c b, passing into the lower half of the slit to be brought to a focus like the hydrogen rays, but in the upper half of the tube. Evidently the two may be photographed at the same time. The ex posure for oxygen may need to be pro longed beyond that of hydrogen, owing to loss of light in travelling through the prism. The extra exposure is easily ar
ranged by interposing a black card to of two pieces of wire of the that is being examined. These are, like; the Geissler tubes, put in series with the secondary of the Ruhmkorff coil, as shown; by Fig. 769. As the current leaps across) from one terminal a to the other b itl vaporises the metal, and the spark isl coloured by the glowing metallic vapour.) There is some disagreement as to thel temperature attained at spark gaps and in Geissler tubes. Some observers the temperature in the tubes at a much higher value than that of the electric] arc. The spark gap must be adjustable,) and, if " long'' and " short'' lines are quired, it must be placed at right angles to the slit length, as in the case of the arc. The terminals, a and b, are often enclosed in a glass tube, which can be intercept the hydrogen rays at the proper time. The spectra for both will be simul taneously in focus, because images of the same slit are being photographed.