SOL RUES OF LIGHT.
It should be noted that there is no danger of displacement of the spectrum lines by changing the source of light, because the image photographed is that of the slit, the source of light being used for illuminating it. To get a continuous spec trum a glowing solid must be used. There are several sources available ; the simplest being an ordinary coal-gas flame. The light, in this case, is due to incandescent carbon particles, but it is not of a good white colour. A Welsbach burner or oxy coal-gas light would be better, but the source most used is the incandescent crater of the electric arc. In this case, the carbons are placed parallel to the slit, with the negative carbon slightly above and in advance of the positive. The crater will then face the collimator end. A scissors type of carbon holder is, perhaps, preferable, as the arc is only in use for a short time, and the adjust ment can be made more delicately. The
temperature of the crater is about 3,500° to 4,000° C. This source of light is con venient, clean, reliable, and yields a good white colour. Compare the appearance of an arc light with that of glow or gas lamps on the street. The former, if not of the modern " flare " type, seems to have a decided bluish-violet tinge, simply because of the preponderance of yellow in the other lights. The arc light con tains a large proportion of violet and ultra-violet rays, thus being especially suitable for photographic work. In using a batswing burner, put the flame edge wise towards the slit. A fair spectrum may be obtained without a slit by covering all but a short portion of an incandescent lamp filament and placing the latter at a fecal length from the collimator lens.