286 Dr. R. A. Houstoun on a New 
latter instrument has shown me*. The latter instrument 
also has the advantage that the polarizing prism is placed 
after the dispersion-prism and the error due to depolarized 
light removed. In practice, however, since twice as much 
light is lost in the latter instrument, the Hiifner spectro- 
photometer is as accurate, and is of course simpler. The 
University of Glasgow possesses a Wild spectrophotometer, 
but so much light is lost in the latter instrument that in 
accuracy it is very much inferior to the others. 
To test the possibilities of the instrument for spectrophoto- 
metry in the ultra-violet, I attempted to measure the absorption 
of copper sulphate photographically. Copper sulphate was 
chosen because it is a salt for which accurate values are 
given in Landolt and Bornstein's tables. It absorbs the red 
end of the spectrum, but the results show that rough 
quantitative work in the ultra-violet could be done very 
easily. 
A nicol was attached to the end of the collimator. The 
diameter of its aperture was less than one quarter the diameter 
of the lens, and hence there was a great loss of light. In 
place of the telescope there was placed a camera, the focal 
length of its lens being about 51 cms. Wratten and Wain- 
wright's panchromatic plates were used. The spectrum from 
6700 to 3500, where the absorption of the glass begins, was 
taken at one exposure and measured more than 5 cms. in 
length. 
First of all both beams were left free and the nicol set so 
that its plane of polarization made angles of 55°, 52°, and 
50° with the plane of polarization of the lower beam. At 
52° the one spectrum was brighter, at 50° the other, so 51° 
was taken to be the angle at which the nicol would be set 
for equal brightness. A photograph was then taken of the 
flame spectrum of Li, K, Na, and Th, and the wave-length 
of a point of the plate determined in terms of its distance 
from the D lines. 
Then the cell with the copper sulphate was placed in 
position, and exposures made for different positions of the 
nicol. A bunsen flame with a sodium bead was placed 
between the incandescent mantle and the cell. The sodium 
line appeared therefore on all the plates. The distance of 
the point on the plate where the two spectra touching one 
another were equally black was measured from the D line, 
and the wave-length of the point could thus be determined. 
* "Untersuchungen liber die Absorption des Lichtes in isotropen 
Korpern," K. A. Houstoun, Ann. der Physik, xxi. p. 535 (1906). 
