670 DR. W. HUGGINS ON THE PHOTOGRAPHIC SPECTRA OF STARS. 
§ II. Apparatus. 
In consequence of the very limited amount of light received from the stars, it was 
obviously of the first importance to spread out the spectrum to the smallest amount 
that would give a sufficiently visible separation of the principal lines to permit of their 
being easily recognised and measured. Another point in this connexion which required 
consideration was whether a slit should be employed. A slit sufficiently narrow to be 
of use for the purposes presently to be mentioned would allow a portion only of the light, 
concentrated by the speculum in the star’s image, to enter the collimator, and would 
therefore greatly lengthen the exposure required to obtain a photograph. Notwith¬ 
standing this serious drawback I determined to use a slit partly for the sake of a purer 
spectrum, and partly on account of the facility which a slit would give to obtain a 
second spectrum for comparison on the same plate with the star’s spectrum. The 
employment of a slit would also make the same apparatus suitable for use upon the 
moon and planets. 
For the material of the prism I selected Iceland spar, as it is very transparent to 
the ultra-violet rays, and has so much higher a dispersion than quartz that one prism 
only would be sufficient. The prism has a refracting angle of 60°, and is cut in a 
plane perpendicular to the axis of the crystal. Such a prism in any one position gives 
single refraction for light of one refrangibility only, but practically the separation of 
the two images through the range of the spectrum which is photographed is too 
small sensibly to affect the results.* The prism is fixed in a position of minimum 
deviation for H. The lenses are of quartz, cut perpendicular to the axis and plano¬ 
convex in form. The lens of the collimator is 1^ inch diameter, 10 inches focal length. 
The lens placed after the prism to form the image on the plate is of the same diameter, 
inches focal length.! 
The form of construction of the spectrum apparatus is shown in the accompanying 
diagram (fig. 1). 
The wooden frame which receives the photographic “backs” is made to tilt so 
as to allow the plate to be brought into a position in which the rays of different 
refrangibility shall be, as nearly as is possible, in focus together upon the plate. This 
* Professor Stokes has permitted me to add the following note, dated January 23, 1875 :— 
“ I have worked out the deviations for a prism of 60° of calcareous spar, the axis perpendicular to the 
bisecting plane of the prism, the line H at minimum deviation and therefore seen single. I have worked 
out the deviations for B with the results : 
Deviation for H ordinary, extraordinary .... 54° 37''76 
Deviation for B ordinary.51° 32’'49 
„ ,, extraordinary.51° 32 ,- 36 
The difference comes smaller than I had expected, only 0'T3 or 8", the spectrum from B to H being over 
3°. For a line half way between B and H the difference would be only a quarter of that, or 2". The 
difference comes out practically insensible.” 
f The prism and lenses were cut for me by A. Hilger. 
