552 



PRINCIPLES OF CHEMISTRY 



the yellow light emitted by the sodium salt depends on a group of light 

 rays having a definite index of refraction which corresponds with the 

 yellow portion of the solar spectrum, having the index of refraction 

 of the Frauenhofer line (strictly speaking, group of lines) D, whilst the 

 salts of potassium give a light in which these rays are entirely wanting, 

 but which contain rays of a red and violet colour. Therefore, if a 

 potassium salt occur in a flame, then, on decomposing the light (from 



a flame coloured by the 

 salt) after passing through 

 a narrow slit by means 

 of a prism, there will be 

 obtained red and violet 

 bands of light situated at a 

 considerable distance from 

 each other ; whilst if a so- 

 dium salt be present a yel- 

 low line will also appear. If 

 both metals simultaneously 

 occur in a flame and emit 

 light, then the spectrum 

 lines corresponding to the 

 potassium and the sodium 

 will simultaneously appear. 

 For convenience in car- 

 Fig. 72A. Spectroscope. The prism and table are covered with rying Oil this form of test- 

 an opaque cover. The spectrum obtained from the flame 



coloured by a substance introduced on the wire is viewed ing, Spectroscopes (fig. 72A) 

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



illuminate the image of the" scale reflected through B by are constructed/* 5 COnSlst- 

 the side of the prism. . ,, ,, ,. 



ing or a retracting prism 



and three tubes directed on 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), 



23 For accurate measurements and comparative researches more complicated spec- 

 troscopes are required which give a greater dispersion, and which 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 entire internal reflection, passes 

 through the upper portion of the same six prisms, and again by an entire internal 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 of 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 Hoscce* 

 Kayser, Vogel, and Lecoq de Boisbaudran. 



