234 Photographs of the Spark Spectra of Elementary Substances. 
the position of their foci something like six inches nearer to the lens than the foci 
of the visible rays, naturally causes the image of the rays to be smaller, hence the 
spectra all decrease in width as the refrangibility of the rays increases; at the 
same time, although perfectly in focus, the lines tend to become nebulous in 
character. 
Standard Spectra. —The standard for all measurements of wave-lengths are the 
lines ot cadmium so accurately measured by M. Mascart. They also serve as standards 
of intensity and of the sensitiveness in the silver bromide film. Hence at the top 
generally of each plate is a photograph taken of cadmium electrodes, in order to 
determine the position of the lines of the other substances and as a test that the 
plate is of a normally sensitive character, that the development is properly carried 
out, the battery power efficient, and the period of exposure of the plate correct. 
Of the period of caposure.—With regard to the length of time that a plate may 
be exposed to the rays of the spectrum, this varies for the same electrodes with the 
prism power and focal length of the lenses, other conditions being constant. With 
different electrodes it varies with their conductivity and volatility. 
Considerable latitude may be allowed in the exposure of even the most highly 
sensitive gelatine bromide films, thus four perfectly good photographs of the cadmium 
spectrum have been obtained on the same plate, and developed at the same time, 
the first had one minute and the last four minutes in the camera. 
The chief lines of the metal were equally good in each, but the faint metallic 
lines and the air lines were strong in the last photograph. 
It is possible with collodion emulsion plates, such as those of the Rev. Canon 
Beechey, to obtain photographs of the principal metallic lines perfectly distinct, 
the air lines being either invisible or so faint as to be unnoticable. A short exposure 
of a very sensitive emulsion and a long continued development secures this effect. 
By lengthened exposure minute traces of foreign metals betray their presence in 
specimens which otherwise appear of perfect purity, thus I have detected indium in 
cadmium, cadmium, tin and lead in indium, silver in gold, and iron in aluminium, 
even after great care and skill had been exercised in the elimination of all foreign 
matter. 
" The following is a table of exposures’ which yielded the photographs which 
accompany this paper :— 
Magnesium, . . 1 minute. Lead, 2 minutes. 
Zinc, : . eee Tellurium, 4 ,, 
Cadmium,. het; Arsenic, . 6 eo es 
Aluminium, B pp Antimony, ( wet; 
Indium, : 0 A gy Bismuth, . . 5 BA 
Thallium, . ; es Tron, D2) 
Copper, Spe Nickel, Dini 
Silver, 3g Cobalt, Boog 
Mercury, . : hots Palladium, 5 SRS ET 
Carbon (graphite), Ae oe Gold, ; ; ROMaEnS; 
Tin, , ° 2: Aluminium, A 
