1912-13.] Some New Types of Spectrophotometer. 
257 
XXIII. — Some New Types of Spectrophotometer on the “Flicker” 
Principle. By James Robert Milne, D.Sc. 
(Read March 21, 1910. MS. received April 27, 1912.) 
In a previous paper * I have described a new form of spectrophotometer in 
which the comparison of relative brightness is effected by bringing the 
two beams accurately edge to edge : the present paper is for the purpose of 
describing some spectrophotometers in which the comparison is effected by 
allowing the two beams to fall alternately on the eye in rapid succession. 
I. It will perhaps conduce to clearness if I refer to a subsidiary matter 
first — namely, the question of the production of a beam of light suitable for 
use in light-absorption measurements. Unless the beam consists of nearly 
parallel rays, j- and has the same intensity at each point of its cross-section, 
it is clear that measurement of the relative brightness of two parts of it, 
one of which has passed through the absorbing substance, will not lead to 
a satisfactory determination of the absorption. The only way of obtaining 
a reasonably parallel beam is by the use of what amounts to a collimator 
having the usual slit replaced by a somewhat large round hole, and hence 
possible variations of method merely turn on different methods of lighting 
up this hole. I find that both the electric arc and limelight are completely 
unsuited for the purpose, the ensuing beam of light from the collimator 
lens usually possessing darker parts which are evident to the eye if the 
beam be allowed to fall on a white surface, and which may be recorded 
photographically on the surface of a sensitive plate momentarily exposed 
to the light of the beam and afterwards developed in the usual way. A 
gas or other flame focussed by means of a lens on the hole of the collimator 
gives only a weak beam of light, unless, indeed, acetylene gas be employed. 
I worked with the latter for some time with quite satisfactory results, but 
it is messy and troublesome, and I have now discarded it in favour of the 
Nernst electric lamp. Before this had been generally introduced, I tried a 
strip of electrically heated platinum placed immediately outside the hole in 
the collimator, and this is very simple and satisfactory, but it requires a 
somewhat heavy current, and cannot safely be used at a temperature 
* “Some General Principles of the Absorption Spectrophotometer, and a New 
Form of Instrument which embodies them,” Proceedings of the Optical Convention , 
p. 178, 1905. 
t In regard to this point see, e.g., Ewan, T., “ On the Absorption Spectra of Some Copper 
Salts in Aqueous Solution,” Phil. Mag. (5), xxxiii. p. 326, 1892. 
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