Analysis by Spectrum-observations . 507 



exactly into focus by altering the position of the scale in the 

 tube C ; and by turning this tube on its axis, it is easy to make 

 the line in which one side of the divisions on the scale lie, 

 parallel with the line dividing the two spectra, and by means of 

 the screw 8 to bring these two lines to coincide. 



In order to bring the two sources of light, D and E, into 

 position, two methods may be employed. One of these depends 

 upon the existence of bright lines in the inner cone of the 

 colourless gas-flame, which have been so carefully examined by 

 Swan. If the lamp E be pushed past the slit, a point is easily 

 found at which these lines become visible ; the lamp must then 

 be pushed still further to the left, until these lines nearly or 

 entirely disappear ; the right mantle of the flame is now before 

 the slit, and into this the bead of substance under examination 

 must be brought. In the same way the position of the source 

 of light D may be ascertained. 



The second method is as follows: — The telescope B is so 

 placed that the brightest portion of the spectrum of the flame of 

 a candle is seen in about the middle of the field of view ; the 

 flame is then placed before the ocular in the direction of the 

 axis of the telescope, and the position before the slit determined 

 in which the upper half of the slit appears to be the brightest ; 

 the lamp E is then placed so that the slit appears behind that 

 portion of the flame from which the most light is given off after 

 the introduction of the bead. In a similar way the position of 

 the lamp D is determined by looking through the small prism 

 and the lower half of the slit. 



By means of the screw e, the breadth of the slit can be regu- 

 lated in accordance with the intensity of the light, and the 

 degree of purity of spectrum which is required. To cut off 

 foreign light, a black cloth, having a circular opening to admit 

 the tube C, is thrown over the prism P and the tubes A and B. 

 The illumination of the scale is best effected by means of a 

 luminous gas-flame placed before it ; the light can, if necessary, 

 be lessened by placing a silver-paper screen close before the 

 scale. The degree of illumination suited to the spectrum under 

 examination can then be easily found by placing this flame at 

 different distances. 



In order to obtain representations of the spectra of caesium 

 and rubidium corresponding to those of the other metals which 

 we have given in our former paper, we have adopted the following 

 course : — 



We placed the tube C in such a position that a certain 



division of the scale, viz. No. 100, coincided with Fraunhofer's 



line " 1) " in the solar spectrum, and then observed the position 



of the dark solar lines A, B, C, D, E, F, G, II on the scale j 



3L2 



