1912-13.] Some New Types of Spectrophotometer. 
263 
Summary. 
First of all there is discussed the best method of obtaining an intense 
beam of parallel rays having each ray of equal brightness, as the produc- 
tion of such a beam is a pre-requisite of all spectrophotometric measurements 
of light absorption. Next there are briefly described three new types of 
“ flicker ” spectrophotometer, the first of these depending on polarisation, 
the second on refraction, and the third on reflection. All three are quite 
successful : which it is best to use depends on the circumstances of the case. 
Finally, three general governing principles are laid down which would 
seem to apply equally to all types of flicker photometer. 
There remains the pleasant duty of expressing my thanks to the 
Trustees of the Carnegie Trust for the various grants which have made 
these somewhat expensive investigations possible. 
Department of Physics, 
University of Edinburgh. 
( Issued separately July 14, 1913.) 
