1899-1900.] Prof. C. G. Knott on Swan’s Prism Photometer. 13 
have made of it, promises to succeed well for comparing the bright- 
ness of the illuminated apertures, may he made by cementing 
together two equal and similar rectangular glass prisms ABC, 
BCD, 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 placed opposite to the faces AC, CD, and 
the observer looks through the face BF. The light transmitted 
through AC, and falling on BC, 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 CD will be totally reflected 
to the eye by the face BC, 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 BC, except 
when the light totally reflected by BC is equal in intensity to the 
sum of the lights transmitted and reflected at E. The spot E will 
then disappear, owing to the whole surface of BC, 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 BC, and to that transmitted by AC, 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 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 subsequent 
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 photometers constructed by Swan 
himself is now among the apparatus of the Physical Laboratory of 
Edinburgh University, having been purchased by Professor Tait 
some years ago along with the best part of Professor Swan’s private 
collection. This photometer is in regular use in the Laboratory. 
