Notices respecting New Boohs. 227 



The advantages of the photographic method are noticed by the 

 author as follows : — "Absolute truth is everything in spectroscopic 

 work ; and the very best draughtsman working with the most per- 

 fect micrometer cannot, even at the expense of a vast amount of 

 labour, equal in accuracy a good photograph of a set of spectral 

 lines." Of course the photographic plate cannot miss or make mis- 

 takes in lines actually presented to it ; and Mr. Capron's work, as 

 already remarked, is very useful as preliminary ; but the accuracy 

 . obtainable by measurement of his photographs is not as great as the 

 author supposes. Indeed a greater degree of accuracy is attained 

 by eye-measurements with a good micrometer. For measurement 

 of the photographs, the reviewer has employed a photographic re- 

 duction (on glass) of a millimetre-scale having about 5 divisions to 

 a millimetre. For example, the wave-lengths of the Aluminium- 

 lines obtained from an interpolation-curve drawn from the air-lines 

 gave 5047, 4660, 4528, 4510, and 4476 ; Thalen has for the same 

 lines 5056, 4662, 4529, 4511, and 4476. And similar reductions 

 of the iron-lines in the selenium-spark gave 4416, 4383, 4407, 

 4323, 4304, 4268, 4257, 4248, 4148, 4072, 4066, and 4048, where 

 Thalen has 4415, 4383, 4404, 4325, 4307, 4271, 4260, 4251, 4143, 

 4071, 4063, and 4045. With a similar direct-vision spectroscope, 

 and the micrometer described in the Number of this Journal for 

 August 1875, closer measurements can be obtained, as is seen by 

 the following comparison : — 



Lines in the Spectrum of burning Magnesium. 

 Direct-vision Six-prism automatic 



spectroscope. spectroscope. 



5006 5007 



4996 4997 



4985-5 4986 



4974-5 4975 



4963-5 4963 



4948-5 4948 



4934 4934 



Still very respectable results may be obtained by careful measure- 

 ment of these photographs. 



There are several points of interest suggested by a study of these 

 "photographed spectra," — -why, for example, some lines of par- 

 ticular metals are plainly marked, while others are absent. For in- 

 stance, in the Lead spark- spectrum the line 4058 is sharp and 

 bright, while there is no trace of the brighter line 4167. 



The photographs obtained from the electric light are particularly 

 interesting. This is, as far as we know, the first extensive series 

 of measurements of spectra obtained by the ignition of substances 

 in the electric arc. In all the photographs are seen, more or less 

 distinctly, certain lines which Mr. Capron terms "point-lines." A 

 few of these are certainly Iron-lines ; but a particular set, employed 

 by Mr. Capron as "arc" reference-lines, are certainly due to Carbon, 

 They are seen in the third photograph of Plate Y., and are the 



Q2 



