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CAPTAIN ABNEY AND LIEUTENANT-COLONEL FESTING 
It appeared to us that the longer waves of the spectrum would be more likely 
to be affected by their passage through complex bodies than the shorter waves, and 
that therefore we ought to look for the absorption effects of the different molecular 
groupings in such bodies at the least refrangible end of the spectrum. The absorption 
spectra of various bodies in the ultra-violet portion of the spectrum have already been 
photographed by Professors Hartley and Huntingdon, and whilst that region seems 
specially adapted for certain series of hydro-carbon compounds, through a diminution 
or otherwise of the general absorption, &c., special or selective absorptions for all of 
them apparently lie at the opposite end of the spectrum. The results which we have 
tabulated in our maps fully support this view, indicating without much doubt that the 
substances we have examined can be grouped according to their absorption spectra; 
and that such grouping, as far as we have examined it, agrees on the wdiole with 
that adopted by chemists. It would be premature, however, to make any general 
deductions, since the ground to be covered is co-extensive with the compounds them¬ 
selves ; and as we have but made a selection which we believe may be regarded as 
typical, it will require the patient labour of many for a long period before this new 
branch of physico-chemical research can be in any way regarded as complete. We 
have thought that, by describing our method of work, and by publishing such results 
as we have already obtained, we might hasten the more extensive research which must 
eventually be undertaken. We have therefore the honour to bring the matter before 
the Society, with a statement that it is our intention as opportunity occurs to continue 
these investigations. 
Apparatus. 
Spectroscope. —The spectroscope employed was an excellently planned one by 
A. Hilger, consisting of three prisms of 62°, and a half prism to which was cemented 
a right-angled prism for the purpose of reflecting back the rays. The glass was white 
flint of medium density. The beam of light underwent total reflection at the half 
prism, so that the battery consisted in reality of one, three, five, or seven prisms, as 
occasion might require. The great advantage of this arrangement is that when once 
the camera is in position it does not require readjustment for any movement or 
alteration in the number of the prisms. There being, however, occasions when we 
rather doubted the performance of this arrangement, we adapted the spectroscope to 
the ordinary form without reflection. The slit of the spectroscope was furnished with 
an arrangement by which every third or half only of the aperture need be uncovered at 
a time. The collimating lens was of 20 inches focal length, and that of the lens fitted 
to the camera was of nearly the same, and in some cases of double that length. The 
dispersion of three prisms from A to D is about 3° 10'. 
Camera .—The camera employed was made by Mr. P. Meagher from designs 
furnished by one of us, and is capable of being employed with any lens whose focal 
length lies between 9 inches and 6 feet. It lias a horizontal swing back, which is 
