256 
MR. J. NORMAN LOCKYER ON SPECTRUM-ANALYSIS 
From the time of Wheatstone’s first experiments, when in 1835 he stated that if the 
poles consisted of two different metals the spectrum contained the lines of both metals, 
down to the researches of Stokes, Miller, and Robinson in 1862, there is no reference, 
so far as I can find, to any localization of light in any portion of the breadth of the 
spectrum. In the case of the spark taken between two poles, e. g. in air, the spectrum 
is generally one in which the lines of the two vapours and of air are blended together, 
all the lines running across the field. 
But under certain conditions this is not so. Thus Stokes*, who used the spark itself 
instead of a slit, remarked that the metallic lines are “ distinguished from air lines by 
being formed only at an almost insensible distance from the tips of the electrodes, 
whereas air lines would extend right across.” 
MiLLERf, who used a slit and a spark close to it, referring to his photographs of 
electric spectra, remarks, “ 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.” 
On the same subject Robinson $ writes, “ At that boundary of the spectrum which 
corresponds to the negative electrode (and in a much less degree at the positive) 
extremely intense lines are seen, . . . which, however, are short.” 
Thalen (though he also did not adopt the method used by Dr. Frankland and 
myself in and since 1869) observed this localization to a certain extent, doubtless on 
account of the long collimator which he employed. 
He remarks § : — “ II y a aussi des raies brillantes qu’on n’observe que dans des cas 
exceptionnels, comme, par exemple, quand la quantite de la substance soumise a 
1’ experience est tres-abondante ou quand l’incandescence devient tres-vive. Ces raies 
qui se presentent ordinairement aux bords du spectre sous la forme de points d’aiguille, 
meme quand les autres raies du metal forment des lignes continues en travel’s du spectre, 
out ete representees sur la planche par des lignes tres-courtes. ” 
Before I proceed further I beg to refer to the two annexed woodcuts (figs. 2, 3), copied 
Cadmium. 
Zinc. 
from photographs of a part of the spectrum observed when the jar-spark passes (1) between 
the poles of zinc and cadmium , and (2) between cadmium and lead , and the image is thrown 
on the slit. It will be seen that in the case of these metallic vapours (and it is true of 
* Philosophical Transactions, vol. clii. 1862, p. 603. f Op. cit. p. 877. + Op. cit. p. 947. 
§ “ Memoire sur la determination des longueurs d’onde des raies metalliques,” p. 12, printed in the Nova 
Acta Regise Societatis Scientiarum Upsaliensis, ser. iii. vol. vi. Upsala, 1868. 
Fig. 2. 
Violet. Red. 
