14 Proceedings of Royal Society of Edinburgh. [sess. 
In the same collection were also two other small prisms intended 
for the same purpose but not made up. The lid of the small box 
containing them still bears the inscription in Professor Swan’s own 
handwriting: — “Pair of fine plate-glass prisms made for me by 
Cooke (1870) for my prism photometer.” This inscription, written 
fully ten years after the first published description, shows that 
Swan was in the habit of using his photometer. 
The fact that Swan had forestalled Lummer and Brodhun in 
the invention and construction of an ingenious form of photometer 
has, of course, been familiar to all officially connected with the 
Edinburgh University Physical Laboratory for some years past. 
Recently, having occasion to inquire somewhat closely into the 
history of photometric methods, I determined to make a systematic 
search through Swan’s published papers, which for the most part 
treat of optical subjects. I had not far to search ; for on the 
plate illustrating the second paper named above I recognised at a 
glance the prism photometer, and immediately thereafter discovered 
the descriptive paragraph. My expectation at most was to find 
some incidental reference to the instrument. To my surprise I 
found as complete a description of the essential instrument as any- 
one could desire to find. It will remain always a matter of no 
small astonishment that such an important contribution to know- 
ledge should have escaped the notice of the myriad workers in 
photometry. In Swan’s day there was not the same great interest 
taken in the subject ; but that is no excuse for present neglect. 
Swan’s photometer was given to a world not ready for its 
reception. Let us now who know its value not forget that it is 
“ Swan’s ” photometer. 
