4 86 HISTORY OF SCIENCE. 



by any other method, and comparable in weight to a 2oo-millionth 

 part of a grain, would readily indicate their presence by their spec- 

 trum. 



To the German professors, Kirchhoff and Bunsen, with whom was 

 also associated in these researches our eminent countryman, Professor 

 Roscoe, science is indebted for the development of spectroscopic ob- 

 servation as a means of chemical analysis. The manner of viewing 

 the spectra of flames devised by Bunsen will be understood by inspec- 

 tion of Fig. 222, on which is represented the arrangement of his first 

 and simplest apparatus. The dark chamber is represented by the box 

 here shown with the cover removed. Near the middle of the box 

 is the prism d. The prism here shown is a hollow one, its sides 



FIG. 222. 



being made of flat plates of glass cemented together at the proper 

 angles, and containing, as in a bottle, some liquid of great dispersive 

 power, such as sulphide of carbon. In one side of the box is fixed a 

 tube, which carries at its outer extremity c an arrangement by which 

 the very narrow vertical opening or slit placed there can be adjusted 

 to any required width. The end of this tube next the prism carries a 

 lens, and as the slit is placed near its principal focus, its action is to 

 render parallel the rays which fall upon the prism, so that with the 

 slit and the prism only a few inches apart, the same effect is obtained 

 as if the slit were at a great distance. The rays, after their passage 

 through the prism, are received into a telescope fixed on another side 

 of the box, and the lines, seen on looking through the telescope, 

 are in fact so many different images of the slit as there are rays of 

 different refrangibilities to produce them. To obtain the spectra of 

 many substances it suffices to place a drop of their solutions on the 

 end of a platinum wire supported by the stand b in the flame of the 



