12 
Proceedings of Royal Society of Edinburgh. [sess. 
On Swan’s Prism Photometer, commonly called 
Lummer and Brodhun’s Photometer. By Prof. 
C. G. Knott, D.Sc. 
(Read December 19, 1899.) 
In 1849 William Swan, subsequently Professor of Natural Philo- 
sophy in the University of St Andrews, read a paper on the 
“ Gradual Production of Luminous Impressions on the Eye and 
other Phenomena of Vision ” before the Royal Society of Edinburgh 
(see Transactions , Vol. XVI.). This paper contains some results 
of high interest, hut I have no recollection of ever having seen it 
referred to in modern literature on the subject. 
On April 4, 1859, Professor Swan gave a second paper on the 
same subject, much briefer than the first, and entirely occupied 
with descriptions of greatly improved forms of apparatus (see 
Transactions , Vol. XXI.). Among the forms of apparatus de- 
scribed is his “ Prism Photometer.” This is simply and solely the 
form of photometer described in 1889, exactly thirty years later, 
by Lummer and Brodhun, and named after them in all recent 
literature (see Zeitschrift fur Instrumentenhunde , Bd. 9). I cannot 
do better than give Swan’s own description in full, and reproduce 
his own diagram. 
He writes : — “ An arrangement, which, from an imperfect trial I 
