246 Prof. Bunsen on Caesium, 



the caesium spectrum ; the second gives the readings on the scale 

 of their apparatus corresponding to these lines ; the third column 

 contains numbers representing the same readings reduced to the 

 scale which is given in the Plate published with our memoir in 

 Fresenius's c Journal of Analytical Chemistry '*. This reduc- 

 tion is founded upon the statement made by Messrs. Johnson 

 and Allen, that on their scale the sodium line lay at 100, and 

 the blue strontium line at 156. On the scale contained in 

 Fresenius's Journal, the left edge of the sodium line lies at 50, 

 that of the blue strontium line at 56 ; hence the numbers of 

 the third column are easily obtained from those of the second 

 simply by subtracting 50. The fourth column contains the 

 positions of the caesium lines from our drawing in PoggendorfPs 

 Annalen reduced to the same scale. 



On comparing the corresponding numbers in the third and 

 fourth columns we do not find any differences of real moment. 

 A more exact agreement can hardly be expected when we re- 

 member that in the absolute position of the lines of the spectrum 

 seen upon a scale such as we used, and as Messrs. Johnson and 

 Allen, as far as can be gathered from their description, appear to 

 have used, unavoidable errors of observation are introduced, 

 which are rendered less evident (owing to the achromatism of the 

 glass lenses and of the eye) by the fact that, in whatever 

 position the apparatus may be placed, only one point on the 

 spectrum can be distinctly read off on the scale. In construct- 

 ing our apparatus with a photographic scale, we did not deviate 

 without good reason from the much more accurate means of 

 measurement used by physicists in the determination of the 

 refractive indices of transparent bodies, inasmuch as the che- 

 mist, for whom our apparatus is specially designed, does not 

 require so much an exact knowledge of the absolute position of 

 the single lines in the spectrum as he needs to be able to observe 

 quickly and easily, especially when lines have to be recognized 

 which only flash out for a moment. The point of greatest im- 

 portance in spectrum-analysis is the degree of certainty with 

 which it is possible to decide whether two lines are identical in 

 position or not ; and this decision is most easily attained by 

 means of the small prism on the slit. 



The slight differences which appear between the numbers in the 

 third and those in the fourth column may further be caused from 

 errors in the interpolation which has to be made in order to reduce 

 the readings of one apparatus to the scale of another. For the pur- 

 pose of comparing their observations with ours, Messrs. Johnson 

 and Allen have reduced their readings to the scale from which 

 the numbers in column 3 are derived, and they have then repre- 

 * Fresenius' Zeitschrift fur Analytische Chemie, 1862, Heft 1, fig. 2. 



