120 Prof. C. Gr. itnott on Swan's Prism Photometer, 



of his earlier results ; and it was not his habit to write for 

 mere writing's sake. 



But whatever may have been the real reason for his sub- 

 sequent silence, there is not the least doubt that Swan invented, 

 described, constructed, and used, thirty years before the 

 scientific world was ready for it, the prism-photometer which 

 Lummer and Brodhun had to re-invent. One of the photo- 

 meters constructed by Swan himself is now among the 

 apparatus of the Physical Laboratory of Edinburgh Uni- 

 versity, having been purchased by Professor Tait some years 

 ago along with the best part of Professor Swan's private 

 co!lection. This photometer is in regular use in the Laboratory. 



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 {date- 

 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 con- 

 nected with the Edinburgh University Physical Laboratory 

 for some years past. Recently, having occasion to enquire 

 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 recognized 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 anyone could desire to find. It will remain 

 always a matter of no small astonishment that such an im- 

 portant contribution to knowledge 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. 



