Prof. C. G. Knott on Swan's Prism Photometer. 119 



trial I have made of it, promises to succeed well for com- 

 paring the brightness of the illuminated apertures, may be 

 made by cementing together two equal and similar rectangular 

 glass prisms A B C, B C D, so as to form a parallelopiped, by 

 means of a small portion of Canada balsam, which, when the 

 prisms are pressed together, expands into a circular thin 

 film E. The illuminated apertures C, D' in the screens are 







c 



<C\ 









F 



el 



7T 















s' 



L 





z 



) \ 



placed opposite to the faces AC, CD, and the observer looks 

 through the face B F. The light transmitted through AC, 

 and falling on B C, will be totally reflected, except the portion 

 which falls on the film of Canada balsam at E, which will be 

 nearly all transmitted to the eye of the observer. The light 

 which is transmitted through the face C D will be totally 

 reflected to the eye by the face B C, except what falls on the 

 Canada balsam at E, which will be nearly all transmitted. 

 The spot E will appear of a different brightness from the 

 rest of the surface B C, except when the light totally reflected 

 by B C is equal in intensity to the sum of the lights trans- 

 mitted and reflected at E. The spot E will then disappear, 

 owing to the whole surface of B C, including the spot, 

 becoming uniformly bright. Assuming that the light partially 

 reflected at E has a constant ratio to that totally reflected by 

 the rest of the surface B C, and to that transmitted by A C, it 

 is obvious that the squares of the distances of the flame from 

 the aperture D' when the spot E disappears will give the 

 ratio of the intensities of the lights transmitted by the 

 aperture C." 



Swan's intention was to publish the results obtained with 

 his improved apparatus ; but we can find no record of the 

 continuation of the work. Probably he obtained nothing 

 that materially added to or in any way affected the accuracy 



