496 Proceedings of Royal Society of Edinburgh. [sess. 
Note on a Method of Bringing Together the Two Spectra 
Compared in the Ordinary Spectrophotometer. By 
J. R. Milne, B.Sc. Communicated by Professor J. G. 
MacGregor. 
(MS. received July 10, 1903. Read July 20, 1903.) 
In Spectro-Photometry one of the chief difficulties in making 
accurate measurements lies in the fact that the eye has to judge of 
the relative intensities of two spectral hands, which, instead of 
having their adjacent edges in contact, have them separated by a 
greater or less dark space. Accordingly, to facilitate a piece of 
spectrophotometrical work in which I am engaged, a Hiifner’s rhomb* 
was tried, which is a special attachment designed to overcome this 
difficulty. On trial, however, I found that by this means the fault 
is not entirely removed, although the separating dark space is by 
this means reduced to a very narrow line. The only alternatives 
to the Hiifner’s rhomb which I know are the devices of Brace, 
and of Lummer and Brodhun. The first of these depends on the 
employment of a particular kind of prism in the spectroscope.! 
The other device, due to Lummer and Brodhun, J places before 
the ordinary spectroscope prism a special photometer cube.§ Both 
these devices, however, suffer from the drawback that they are 
unsymmetrical as regards the two beams of light, whose relative 
intensities it is our object to measure. The purpose of this note 
is to explain a method which, while presenting other advantages, is 
free from this defect. In figure 1 let P P' be the plane in which 
fall the two spectra to be compared with one another. These spectra 
are formed in the ordinary course by the object-glass of the telescope 
of the spectrophotometer (which is not shown in figure). When 
* Zeitschr. /. physik. Chem., 3, 562, 1889. 
t See the Astrophy steal Journal , xi. 6, 1900. 
t See Zeitschr. f. Instrk., ix. 23, 41, 461. 
§ Originally invented and described by Swan. See Proc. R.S.E . , vol. xxiii. 
p. 14, and Trans . of same, vol. xvi. 
