PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 2 NOVEMBER 15. 1916 Number 11 
PATH DIFFERENCES WITHIN WHICH SPECTRUM INTER- 
FERENCES ARE OBSERVABLE 
By Carl Barus 
DEPARTMENT OF PHYSICS, BROWN UNIVERSITY 
Received by the Academy, September 9. 1916 
1. Reflecting systems. — Certain earlier results (Carnegie Publications, 
No. 249, 1916, §56) made it seem plausible that the path differences 
within which interferences are obtained (i.e., the apparent lengths of 
uniform wave trains) increase as the dispersion, to which the incident 
collimated white light (L figure 1) is subjected, is made continually 
greater. With this quest in view the aim is to produce the inter- 
ferences by one and the same method, but Vvith a successive variation 
of the dispersion of the spectra. The method (figure 1) was first selected 
for this purpose (M, N, P\ mirrors; T, telescope; s,s', screens), inasmuch 
as the use of prisms or gratings of different dispersive power at P meets 
the requirements, while spectra of the first and second order are equally 
available. 
In work of this kind the spectra must be bright; otherwise the fine 
lines will escape detection. Deficient values will thus be obtained if 
the spectra are too dark. Moreover the results can not furnish data 
of precision, since the exact instant at which fringes, continually de- 
creasing in size, have actually vanished, can not be fixed; and it is the 
fine fringes which furnish a considerable amount of the displacement. 
The differences, however, are so large, that orders of values are appar- 
ent, more than sufficient to substantiate the argument. 
It is possible that the method (figure 1) gives the half ranges only, 
since the efficient pencils of light, C C\ can not cross each other when 
M is displaced. The methods applied will nevertheless be trustv/orthy, 
since they are identical, the same telescope and other appurtenances 
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