POTASSIUM, KUBIDIUM, CJESIUM, AND LITHIUM 



561 



For convenience in carrying on this kind of testing, spectroscope^ 

 (fig. 72) are constructed, 23 consisting of a refracting prism and three 

 tubes placed in the plane of the refracting angle of the prism. One 

 of the tubes, C, has a vertical slit at the end, giving access to the light 

 to be tested, which then passes into the tube (collimator), containing a 

 lens which gives the rays a parallel direction. The rays of light having 

 passed through the slit, and having become parallel, are refracted and dis- 

 persed in the prism, and the spectrum formed is observed through the 

 eye-piece of the other tele- 

 scope B. The third tube 

 D contains a horizontal 

 transparent scale (at the 

 outer end) which is 

 divided into equal divi- 

 sions-. The light from a 

 source such, as a gas 

 burner or candle placed 

 before this tube, passes 

 through the scale, and 

 is reflected on that face 

 of the prism which stands 

 before the telescope B, 

 so that the image of 

 the scale is seen through 

 this telescope simul- 

 taneously with the spec- 

 trum given by the rays 

 passing through the 

 slit of the tube C. In 

 this manner the image of the scale and the spectrum given by the' 

 source of light under investigation are seen simultaneously. If the 



23 For accurate measurements and comparative researches more complicated spec- 

 troscopes are required which give a greater dispersion, and are furnished for this 

 purpose with several prisms for example, in Browning's spectroscope the light passes 

 through six prisms, and then, having undergone an internal total reflection, passes 

 through the upper portion of the same six prisms, and again by an internal total reflec- 

 tion passes into the ocular tube. With such a powerful dispersion the relative position 

 of the spectral lines may be determined with accuracy. For the absolute and exact 

 determination ot the wave lengths it is particularly important that the spectroscope 

 should be furnished with diffraction gratings. The construction of spectroscopes des- 

 tined for special purposes (for example, for investigating the light of stars, or for deter- 

 mining the absorption spectra in microscopic preparations, &c.) is exceedingly varied. 

 Details of the subject must be looked for in works on physics and on spectrum analysis. 

 Among the latter the best known for their completeness and merit are those of Boscoe, 

 Kayser, Vogel, and Lecoq de Boisbaudran. 



FIG. 72. Spectroscope. The prism aud table are covered with 

 an opaque cover. The spectrum obtained from the flame 

 coloured by a substance introduced on the wire is viewed 

 through B. A light is placed before the scale D in order 

 to illuminate the image of the scale reflected through B 

 by the side of the prism. 



