. The book of photography; practical, theoretical and applied. ube. Evidently the two may bephotographed at the same time. The ex-posure for oxygen may need to be pro-longed beyond that of hydrogen, owing toloss of light in travelling through theprism. The extra exposure is easily ar-ranged by interposing a black card to of two pieces of wire of the substancethat is being examined. These are, likethe Geissler tubes, put in series with thesecondary of the Ruhmkorff coil, as shownby Fig. 769. As the current leaps acrossfrom one terminal a to the other b itvaporises the metal, and the spark iscol
. The book of photography; practical, theoretical and applied. ube. Evidently the two may bephotographed at the same time. The ex-posure for oxygen may need to be pro-longed beyond that of hydrogen, owing toloss of light in travelling through theprism. The extra exposure is easily ar-ranged by interposing a black card to of two pieces of wire of the substancethat is being examined. These are, likethe Geissler tubes, put in series with thesecondary of the Ruhmkorff coil, as shownby Fig. 769. As the current leaps acrossfrom one terminal a to the other b itvaporises the metal, and the spark iscoloured by the glowing metallic is some disagreement as to thetemperature attained at spark gaps andin Geissler tubes. Some observers putthe temperature in the tubes at a muchhigher value than that of the electricarc. The spark gap must be adjustable,and, if long and short lines are re-quired, it must be placed at right angles,to the slit length, as in the case of the|arc. The terminals, a and &, are oftenenclosed in a glass tube, which can be. Fig. 7GS.—Projection of Solak Eais on Collimator Slit. intercept the hydrogen rays at the propertime. The spectra for both will be simul-taneously in focus, because images of thesame slit are being photographed. Arc and Sun. Another method is available with arcspectra. A heliostat is arranged so as toreflect sunlight between the carbons. Oncovering different halves of the slit, one slit with two sliding doors, andexposing separately for each source, twospectra, one due to the arc and the otherdue to the sun, will be obtained side byside. By the arrangement shown in both sources of light diverge frombetween the carbons, and the same adjust-ment of the camera is, of necessity, accu-rate for both images. Spark Spectra of Metals. A very simple method of observing thespectra of metals is to make a spark gap filled with other gases than air, to ex-amine the effect of these on the pressure of the enclos
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