1903-4.] Mr J. E. Milne on a New Form of Juxtapositor. 355 
A New Form of Juxtapositor to bring into Accurate 
Contact the Edges of the two Beams of Light used 
in Spectrophotometry, with an application to 
Polarimetry. By J. R. Milne, B.Sc., Carnegie Scholar 
in Natural Philosophy. 
(Read June 20, 1904. MS. received June 23, 1904.) 
In the ordinary spectrophotometer and in Laurent’s “half- 
shade ” polarimeter, two neighbouring patches of light of the 
same colour hut of different intensities are presented to the eye 
of the observer, •who by an appropriate means reduces the 
intensity of the brighter until in his judgment it is brought down 
to the same intensity as the other. The accuracy of such a 
measurement must depend on two factors. The first factor is the 
accuracy with which the observer’s eye can judge of the equality 
of the two patches of light, and the second factor is the accuracy 
with which the instrumental reading indicates the intensity of 
the comparison beam, i.e., of the beam whose brightness is reduced 
till it becomes equal to that of the other. Now it is found 
that in ordinary cases the error of the eye’s judgment in such 
measurements amounts to about 4% or 5%, while the measurement 
of the instrumental regulation of the light can be made much 
more accurately. Accordingly, the error in the measurements 
made with a spectrophotometer cannot be much less than 4% or 
5% unless some special means be employed for improving the eye’s 
power of judgment in such a case, and the mere provision of a 
finer instrumental graduation will not meet the difficulty. Con- 
siderable assistance would be rendered to the eye were the two 
patches of light, whose equality the eye is to judge, brought with 
their edges accurately to touch each other so that no hiatus existed 
between them. As a rule however such a hiatus does exist, for 
should the two lights be from different sources, the edge of the 
mirror or other appliance which directs the comparison beam into 
