“On a New Absorptiometer” bv Arthur Schuster, 
Ph.D, 
In some recent researches Professor Vogel found that the 
relative intensity of the red and blue part of the solar 
spectrum was subject to great changes. While working 
with the spectroscope at considerable heights on the southern 
slope of the Western Himalayas, I was struck by the same 
fact. The instrument which I have now the honour to 
exhibit before the Society is constructed in order to measure 
the relative intensity of the red and blue light in the solar 
or any other spectrum, by comparing the intensity of each 
ray with that given out by a standard lamp. The photo- 
metric principle involved in the measurement is that first 
used by Professor Zollner. The intensity of a certain part 
of the spectrum is brought to the same intensity as that of 
the standard light by a system of Nicol’s prisms. Pro- 
fessor Zollner only compared the whole intensity of two 
sources of light and did not investigate the relative intensity 
of the different colours. D. Glau constructed another ap- 
paratus by which he could measure the relative intensity of 
different colours, but his instrument was constructed for an 
entirely different object, and is not suitable for the purpose 
for which the present instrument is made. 
The instrument, which I have called absorptiometer, 
because it is intended chiefly for the determination of the 
absorption of light taking place in our atmosphere, consists 
of a table similar to that of a goniometer table, but being 
able to turn round on a horizontal axis so as to give it any 
inclination to a horizontal surface. The telescope of the 
goniometer is replaced by a direct -vision spectroscope. 
Opposite the spectroscope a tube is fixed to the table con- 
taining two Nicol’s prisms. One of the prisms is fixed, 
the other can be turned, and its azimuth read off on a 
graduated circle. The standard light is placed behind its 
tube. The intensity of the light falling unto the slit of the 
A 
spectroscope is — sina, where a is the angle between two of 
the principal planes of the two Nicol prisms, and A the 
