Chap, xxvi.] THE SPECTROSCOPE. 325 



receive them, and form a vertical image. Now a gas 

 lamp placed in front of the slit will give a continuous 

 spectrum, or a sodium flame may be brought in front 

 of the slit, and so on. 



A single prism cannot give very great dispersion. 

 If, therefore, great dispersion is wanted a train of 

 prisms is made use of. The second prism is so placed 

 that it receives the rays refracted by the first, and 

 increases the divergence : the third is so placed that 

 it receives the refracted rays from the second and still 

 further disperses them, and so on. A considerable 

 number of prisms may be used. They require, of 

 course, to be arranged in a curve in order that one 

 prism may catch the rays from the preceding one, and 

 the telescope is placed so as to catch the rays from the 

 last. With such an arrangement a spectrum of great 

 length may be obtained. Many spectroscopes have a 

 third tube, which carries at the outer end a small 

 transparent scale. A candle illuminates the scale. 

 At the other end of the tube is a lens. This tube is 

 so placed that the light from the scale falls on the 

 surface of the prism next the telescope and is reflected 

 into the telescope. On focussing, an image of the 

 scale may be seen in the telescope. Thus in the same 

 field of view one may have both a scale and a spec- 

 trum, and may determine the position of any band in 

 a spectrum by means of the scale, so aiding in the 

 comparison of different spectra. 



It is often of great advantage to have in the same 

 field both the solar spectrum and the spectrum of the 

 particular substance under examination. For this 

 purpose a small rectangular prism of glass is placed 

 directly in front of the lower part of the slit. Rays 

 of light from a source at the side penetrate this prism, 

 and undergo reflection at one of the internal faces, so 

 that the light is directed through the slit on to the 

 upper part of the prism, and produces a spectrum. The 



