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PHYSICS: C. BARUS 
INTERFERENCE OF PENCILS WHICH CONSTITUTE THE 
REMOTE DIVERGENCES FROM A SLIT 
By Carl Barus 
Department of Physics, Brown University 1 
Communicated, March 18, 1918 
1. The sharp prism heretofore 2 used for cleaving the rays issuing from a 
collimator (or the slit simply) was dispensed with and the endeavor made 
to obtain two rays capable of interference. The assemblage of apparatus is 
shown in figure 1, where S is the slit (to be replaced by a Nernst or a tungsten 
filament), m and n the opaque mirrors, pp' the half silvered plate. The 
rays dd', from S, pass after reflection into c and c ( and may be observed by 
spectrotelescopes placed either at T or T'. In the first experiments the dis- 
tance dp' was about 4 meters and the distance mn, 10 cm. The mirrors n 
and pp' were on micrometers with the screws normal to their respective faces. 
The distance mn must be within the limits of the wedge of light from the slit 
and is therefore small, unless d is very large. Both pp' and n are on the 
