﻿New Method for Mapping the Spectra of Metals. 379 



adjacent portion of the stem to be converted into a closed 

 vessel by means of a flat disk of glass closing the bore, the 

 effect on the change of the volume so enclosed could hardly 

 differ appreciably from that occurring previous to the closure, 

 supposing the bore to be fine. Also if the bulb be nearly 

 spherical the pressure over its surface would seem, so far as 

 change of volume is concerned, to be replaceable without 

 serious error by a uniform pressure equal to that actually 

 found at the level of the bulb's centre. 



We should thus conclude that the change in the volume of 

 a nearly spherical bulb follows approximately the same law 

 as if the bulb were closed and subjected to uniform pressure 

 equal in intensity to that occurring at the level of its centre 

 of gravity. A relation equivalent to (3) thus seems likely to 

 hold approximately for the bulb alone, at least when it is 

 nearly spherical and the bore is fine. This is, I think, prac- 

 tically in harmony with the conclusion reached by Dr. 

 Guillaume on his p. 111. 



XLIII. On a Neiv Method for Mapping the Spectra of Metals. 

 By Prof. Henry Crew and Mr. Robert Tatnall * 



THE difference in physical character between the various 

 lines in the spectrum of an element has recently assumed 

 such importance that a table of wave-lengths is now, to some 

 extent, incomplete unless accompanied by a photographic 

 map. This is especially true for one who is seeking new 

 relations among the wave-lengths. Thus, in the case of 

 cadmium, the triplets overlap, but, " owing to the physical 

 similarity of the lines forming any one triplet, it is a matter 

 of perfect ease to select them "f. 



Indeed, in many cases where series have been discovered, 

 one might decide to what series a given line belongs quite 

 as well by its appearance as by its wave-length. Rydberg 

 has happily suggested, for these series, names which describe 

 the appearance of their respective lines. 



So far as we are aware, all photographs of metallic spectra 

 which have hitherto been made are, with two exceptions, 

 either of spark spectra or spectra of substances vaporized in 

 the carbon arc. The two exceptions to which we refer are, 

 first, the well-known spectrum of iron by Kayser and Runge, 

 in which the arc employed is that between iron rods about 

 one centim. in diameter ; and, secondly, a copper arc with 



* Communicated by the Authors, 

 t Ames, Phil. Mag. July 1890, p. 45. 



