PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volumes MAY 15, 1919 Number 5 
SUPERPOSED OR DUPLICATED SPECTRUM FRINGES* 
By Carl Barus 
Department of Physics, Brown University 
Communicated, February 17, 1919 
1. Introductory. — To obtain sharp spectrum fringes it is necessary, as a 
rule, to use a slit narrow enough to show the Fraunhofer lines. Hence there 
is sometimes a deficiency of light from this reason alone. It occurred to me 
on producing identical fringes of inclination (achromatics or monochromatics) 
and of color (dispersion), that by their superposition a slit of any width (or 
an entire absence of sht) would be admissible, without destroying the fringes 
in the impure spectrum resulting. 
Furthermore if the edge of the prism is rotated around the axis of the spec- 
tro- telescope 180°, the inclination of all spectrum fringes must be symmetri- 
cally reversed; i.e., inclination up toward the right (positive) will become 
inclination down on the right (negative) to the same amount. The identical 
result may also be reached independently by displacing one of the mirrors of 
the interferometer parallel to itself (path difference) until the fringes passing 
through their maximum size reach the opposed inclination and size. Hence 
there must be a relation of a periodic kind between the displacement of mirror 
(A N) and rotation of the spectro-telescope {A(p), by which sharpness of fringes 
in the absence of a slit is conditioned. 
This device of locating an angle of rotation of the telescope by sharpness 
of fringes, may possibly be used for other purposes something after the 
manner of the half shade or the sensitive tint; for if small, they jump suddenly 
out of an intensely brilliant unbroken spectrum band, when a definite A(p is 
reached. 
Finally, as the fringes are examples of interference of intense non-reversed 
spectra, they should be available in such experiments as described in my last 
paper, for instance. 
*Advance account, from a Report to the Carnegie Institution of Washington. 
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