658 
CAPTAIN W. DE W. ABNEY ON THE PHOTOGRAPHIC METHOD OF 
tion of such an emulsion the water bath must be kept at a temperature but little 
above that of the boiling point of ether. 
Apparatus employed. 
One of the objects I have had in view was to compare the spectra obtained by this 
photographic method with the thermographs obtained by Sir John Herschel, and also 
with the energy curves obtained by means of the thermopile. To do this it seemed 
necessary to work under the same conditions as those under which these results 
were obtained, and I attempted to do so. After many experiments, however, I found 
that the absorption lines lay so close together when the beam of light was sent through 
one prism that I have employed three prisms to obtain greater dispersion. The fine 
prisms that were actually employed were of white flint glass of an angle of 62°, worked 
by Mr. Adam Hilger. The length of the side of the prism is about 2f inches, and 
the height If. A motion for setting and keeping the prisms at the angle of minimum 
deviation for all rays was also made for me by the same optician ; the fact that rays of 
very great comparative wave length were to be examined rendered this all the more 
necessary. The collimator was 20 inches long, and the focus was adjusted for the 
lowest visible ray. The slit had jaws, both movable, which could be opened to any 
required extent by means of a differential screw motion. The lens at first attached to 
the camera had a focus of 18 inches, but subsequently this was abandoned for a lens 
of 30 inches focus, and of about 3 inches aperture. A rather imperfect heliostat, but 
one which sufficiently answered my purpose, was used to cast the beam of fight on a 
condensing lens of 6 feet focus and 5 inches aperture, to form a solar image on the slit. 
The focusing was rather a matter of guesswork, and trial plates had to be exposed to 
attain really sharp images in any part of the ultra-red. The rapid alteration in the 
focus of this portion of the spectrum made it impossible to obtain anything except a 
narrow strip on which the Fraunhofer lines were absolutely sharp, and that only on 
the portion of the spectrum near A, for at the wave length 10,000 the spectrum 
became so compressed that any collection of fine absorption fines inevitably appeared 
as more or less shaded bands. A reflecting mirror was finally adopted in lieu of the 
camera lens, the position and management of which will be more fully described imme¬ 
diately. The difficulty of focussing was thus mitigated, if not altogether surmounted. 
For the photography of the diffraction spectrum the same collimator and camera, 
with its lens, were used, substituting for the prisms a diffraction grating by Rutherfurd 
of about 8600 lines to the inch. A large number of photographs have been taken of 
the ultra-red end of the spectrum with this apparatus, most of which have proved 
useless, in consequence of the difficulty in obtaining an accurate focus of the absorption 
lines. During the past year a different arrangement has been adopted, which is better 
adapted to the diffraction spectrum. 
The following is a sketch of the apparatus as employed ;— 
