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PROFESSOR W. N. HARTLEY OX SPECTRUM PHOTOGRAPHY 
It has been shown by M. Lecocq de Boisbaudrax that when the temperature of a 
source of light (flame or spark) is increased, the relative intensity of the more 
refrangible rays is much increased ; the absolute brilliancy of the less refrangible rays 
sometimes undergoes a diminution which may even amount to extinction (Comptes 
ftendus, vol. lvxxiii., p. 943). It must not be inferred that photographs of spark 
spectra produced precisely in the manner here described are liable to variations in the 
relative intensities of their lines or the order in which they disappear as the quantity 
of substance in the spark decreases. I have observed the invariable character of the 
cadmium, tin, lead, and magnesium lines in about five thousand photographs, including 
not fewer than two hundred examples of other metals, all being obtained for various 
purposes in the course of seven years’ work under such variable conditions as may be 
introduced by the electrodes being near together or far apart, or by the use of a large 
or small coil, but with a condenser of the same size always in circuit. The reason of 
this constancy is sufficiently obvious when we consider that unless the spark be 
almost at the highest temperature attainable the emissive power is insufficient to 
affect the sensitive plate in the usual period of time ; when there is a slight fall 
in temperature there is a shortening of all lines such as is caused by a diminished 
period of exposure. 
The method of using these tables. 
The scale numbers given in the first column of the tables are linear measurements 
of the positions of the lines in the different prismatic spectra, photographed copies of 
which have been published in the Journal of the Chemical Society, vol. xli., p. 84. 
They serve two purposes, first as a check upon the wave-lengths quoted, and secondly 
as a means of identifying the lines. Suppose, for instance, I wish to identify the line 
in the indium spectrum which is mapped as double, it will be seen that the least 
refrangible line stands at nearly 16, and the next at as nearly as possible 40 on the 
scale. Applying an ivory rule to the photograph {loc. cit.), the rule being divided 
into hundredths of an inch, so that the numbers 16 and 40 correspond with the two 
least refrangible lines, it will be found that a very strong line stands at 120, and 
it is this which appears as double on the map, though the fact cannot be seen by 
examination of the printed photographs. A reference to the table will show that at 
119*31 and 119*6S on the scale there are two lines with wavedengths 3257‘8 and 
3255'5, the former of which does not appear in solutions containing ygtli per cent, of 
the metal, while the latter continues visible even when only ywoth per cent, is present. 
The tables of scale numbers and wave-lengths are of little value, however, without the 
maps. As an illustration of the way in which the maps may be used, let us suppose 
that a sample of pyrites cinder is being examined for copper and silver. It will be seen 
at a glance that the most persistent group of characteristic lines in the spectrum of 
silver lies between wave-lengths 2300 and 2500, those lines situated in a position 
