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PROFESSOR W. 1ST. HARTLEY AND MR. W. E. ADENEY 
Cd 18 (\=3402'9 and 2572 - 2), and the third Cd 16 and Cd 26 (\=2836T and 
21457). Between these limits all the lines were accurately focussed. It will be seen 
that these plates overlap, so that on one series of photographs a certain number of 
lines may be measured twice over if necessary. The accurately measured portions of 
the spectra were 4 to 5 inches in length on each plate, so that the whole spectrum 
extended from 12 to 15 inches, and each inch was easily divisible into 10,000 parts. 
The measurements of the lines were made when the cross wires coincided with or 
equally divided the ends of the lines. This is necessary, as many lines are extremely 
short and cannot be measured at any other point. Lines continuous from pole to pole 
are comparatively few in number. “ Ghosts ” of very strong lines appear in the 
photographs of diffraction spectra; they are generally easy of recognition, but should 
it happen that by reason of a crow r d of lines they are not easily distinguishable, they 
may be eliminated by comparing the diffraction with the prism spectrum. 
No metallic line which is not common to both spectra has been measured. It is 
difficult to identify the lines rendered by a prism spectroscope when the original 
photographs only are examined, on account of the necessity of employing the 
microscope, which enables one to view only a small portion of the spectrum at one 
time. For the convenience of identifying the lines and registering their wave-lengths 
two sets of enlargements have been used, each containing about eight spectra, 36 
inches in length. For the purpose of registering the wave-lengths of the air-lines and 
the very numerous lines of iron, enlargements 8 feet in length have been made. 
Determination of Wave-Lengths. 
Method of Working .—The determination of the wave-lengths of the lines in any 
photograph becomes very simple if we know the value of their linear positions on the 
plates in terms of the scale of the goniometer, and so be in a position to find their 
deviations. M. Cornu has described a method by which he determined the deviation 
of some of the lines in his photographs of the ultra-violet solar spectrum. After 
photographing a spectrum he moved his plate so as to obtain an impression of the 
image of the slit on the sensitised film on each side, and very close to the line he 
wished to measure. The points on the arc to which the images corresponded being 
known, the deviation of the line could be determined from them, since the images 
were sufficiently close together for linear distances between them to be taken as 
proportional to angular distances. We have followed the principle of this method. 
As stated in the introduction to this paper, after photographing a spectrum, the 
grating was moved so as to reflect on to the sensitised film a series of images of the 
slit, corresponding to equi-distant fixed points on the arc of the goniometer. In this 
way the spectrum was obtained, together with a number of images of the slit, disposed 
at regular intervals along its length, the images serving as fiducial lines, the deviations 
of which were known with all the accuracy afforded by the scale of the goniometer. 
