the Lines of various Spectra. 43 



Their great similarity has often been noted ; but, as the spark- 

 spectra have nearly always been studied, the exact correspon- 

 dence between them has so far escaped notice. It is to be 

 regretted, too, that in general so little attention has been 

 paid to the physical properties of the lines, when homologous 

 groups or series were selected. It is only, I think, by means 

 of the physical similarities that correct inferences can be 

 made. For these two reasons a great many of the relations 

 given by Prof. Hartley are inaccurate. All of my measure- 

 ments and descriptions of the zinc and cadmium spectra are 

 taken from the photographs of the arc-spectra recently made 

 by Prof. Rowland. My measurements are only provisional 

 and do not possess a greater accuracy than O'l of a unit (t. e. 

 0'01/bbfi). This is sufficient for present purposes, and Prof. 

 Rowland, who has nearly completed a systematic study of the 

 spectra of the elements, will soon publish his tables of wave- 

 lengths, which will be accurate to O'Ol of a unit. There will 

 be noticed a discrepancy between my wave-lengths of cadmium 

 and those given by Mr. Bell* The reason for this was dis- 

 covered by Mr. Bell and myself, when w T e were working on 

 the spark-spectrum of cadmium two years ago. A mistake of 

 one unit in one of Mr. Bell's micrometer-readings vitiated one 

 of the standard wave-lengths (which were used for the de- 

 termination of the others). The scale of my wave-lengths is 

 that of Rowland's map, as its superiority is now universally 

 acknowledged. The wave-lengths are afterwards corrected for 

 atmospheric refraction by means of interpolations inKetteler'sj 

 determinations. These corrections become very uncertain 

 in the extreme ultra-violet : but no better values are known. 



Zinc. 



Wave-length Wave-length in Wave . mimbevs . 

 m Air. vacuo. 



6362-5 63644 1571*2 ±-02 



4810-7 r. ] 4812-2 f 20780+ -04 



4723-7 \ 2117-0 Strong and sharp. 



4680-3 r. J 4681-7 [2136-0 



4630-1 4631-5 2159-1 -h -05 



3346-2 ] ^ 3347-3 / f 2987-5 ±-09 



3345-7 [ | 3346-8 j \ 2987-9 v 



33451 r. J! 3346-2 12988-5 Very strong and 



3303-1 1 [P' 3304-2 ) f 3026-4 nebulous. 



3302-8 r. [ | 3303-8 | \ 3026-8 



3282-35 r. J 3283'4 I 3045-6 



* Amer. Journ. Sci. [3] xxxi. (1886). 

 f Phil. Mag. [4] xxxii. (1866). 



