484 
MR. J. NORMAL LOCKTER ON SPECTRUM-ANALYSIS 
petit which I was already familiar with en grand, and the smaller variations in the 
phenomena enabled me to understand them better. 
I found that in the gold-copper standards an increase of a 1000th part in the gold 
brought the lines down, while a similar increase in the copper carried them up, i. e. 
increased the height of the vapour from the pole. 
I found, on the other hand, that in the silver-copper standards an increase of a 1000th 
part in the silver carried the lines up, while a similar increase in the copper brought 
them down in the field of view, i. e. reduced the height of the vapour from the pole. 
After registering these facts, I saw at once that all the phenomena might be explained 
by assuming a change of volatility ; by assuming, in fact, that alloys differing a 1000th 
part are different physical things, and that the spark acts upon the alloy as a whole as 
well as upon each vapour separately. 
Thus in the cases referred to, in which copper is common to both, we find the 
melting-points to be as follows 
Gold . . . 1200° (Pouillet). 
Copper between 1200° and 1000°, precise point not determined. 
Silver . . . 1000° (Pouillet). 
And the intermediate position which copper occupies at once explains the different 
actions on its lines brought about by the addition of gold and of silver. 
II. THE METHOD OE PHOTOGRAPHING SPECTRA. 
The camera employed in the attempts already made is one for the use of which I am 
indebted to Lord Lindsay. It is furnished with a 3-inch lens by Dallmeyer, of about 
23 inches focal length, and carries a plate 5x5 inches. The camera has replaced the 
observing-telescope of the spectroscope described in my former paper. Three prisms 
of 45° and one of 60° have been employed. 
With these arrangements the whole spectrum from beyond H to the red falls upon 
a 5-inch plate. In order to show how abundantly qualified the lens used is for its work, 
I may state that the solar spectrum from w. 1. 3900-0 to 4500-0 has been obtained at once 
in tolerably fair focus. 
In order to obtain photographs of the solar spectrum of the very best kind, it is neces- 
sary to limit the beam passing through the prisms to very small dimensions — a method 
employed with such admirable results by Mr. Rutherfurd. 
In the attempts to photograph the long and short lines of metallic spectra, it was 
found that this object could not be well obtained with the electric lamp in its usual 
position (with vertical poles), as the central column of dense vapour, as a* rule, extended 
across the arc, i. e. from pole to pole, and gave all the short lines. 
In order to obviate this I determined to make use of a horizontal arc. This was 
accomplished by placing the lamp on its side and firmly securing it in that position. 
The image of the horizontal arc was then thrown on the vertical slit in the usual 
