XL] FIRST GLIMPSES. 147 



generally melts, and we see the shape of the lines varying on 

 the screen as the melting of the slit goes on. 



I began work on this method in 1869, but long before this 

 other observers had obtained a glimpse of some of the pheno- 

 mena, so obvious when it is employed. 



Thus Dr. Gladstone, in I860, 1 in the course of some experi- 

 ments with the light produced by the passage of an electric 

 current through an interrupted stream of mercury, remarked 

 that "the more intense lines appeared broad on account of 

 irradiation, and of greater extent because the slightly luminous 

 environment of the spark gives a perceptible amount of those rays." 



This observation was followed up in 1862 by Professor Stokes, 2 

 who used the spark itself instead of a slit, and remarked that 

 the metallic lines are "distinguished from air lines by being 



FIG. 65. Copy of Dr. Miller's spectrum of cadmium, showing the " dots." 



formed only at an almost insensible distance from the tips of 

 the electrodes, whereas air lines would extend right across." 



Miller, 3 who used a slit and a spark close to it, referring to 

 his photographs of electric spectra, remarked, ' The marginal 

 extremities of the metallic lines leave a stronger image than 

 their central portions," and the extremities of these interrupted 

 lines he terms "dots." 



The accompanying woodcut of the spectrum of cadmium, as 

 photographed by him, will show the expressiveness of the word, 



On the same subject Eobinson 4 writes, "At that boundary of 

 the spectrum which corresponds to the negative electrode (and 



1 Phil. Mag. s. 4. vol. xx p. 250. 



' 2 Philosophical Transactions, vol. clii. 1862, p. 603. 



a Op. cU. p. 877. Op. cit. p. 947. 



I, 2 



