614 
PHYSICS: a BARUS 
NON-REVERSED SPECTRA OF RESTRICTED COINCIDENCE 
By Carl Barus 
DEPARTMENT OF PHYSICS. BROWN UNIVERSITY 
Received by the Academy, September 15. 1916 
In figure 1 the white ray L from the colKmator is diffracted by the 
grating G and the two spectra a and a' thereafter reflected by the 
parallel opaque mirrors M and N, to be again diffracted by the grating 
G' . The rays are observed by a telescope at T. If the gratings, G, G' 
have the same constant, it is ob- 
vious that the field of the tele- 
scope will show a sharp white 
image of the slit, for each mirror. 
li M N G G' are adjusted for 
symmetry by aid of the adjust- 
ment screws on each and the rul- 
ings are parallel, the two white 
slit images will coincide horizon- 
tally and vertically. If now a 
direct vision spectroscopic prism, 
or a direct vision prism grating 
G" is placed in front of the tele- 
scope, the superposed white sHt 
images will be drawn out into 
overlapping non-reversed spectra, 
which will usually show a broad 
strip of interference fringes.^ 
When first found, the fringes 
were very fine parallel lines, fill- 
ing an irregular strip in the orange-yellow region, and it was already 
obvious that an enormous play of the micrometer screw at M would be 
permissible. 
A number of film gratings were tested and the best samples selected 
(Z) = 175 X 10~^), although the dispersion was still too large and 
the D lines not clear. To secure more fight, a beam of sunhght 15 cm. 
in diameter was condensed to a focus on the sKt by a large lens of about 
2 meters in focal distance. The illumination was now adequate and 
the fringes were found at once. These fringes, in view of more accurate 
adjustment, were also larger than before. 
Figure 1 shows, if ah, a'h' and cd, c'd! are pairs of corresponding rays 
FIG. 1. 
