PHYSICS: C. BARUS 
617 
in this way, which are naturally much more sensitive. Size may also 
be changed by compensators and this method is usually more available. 
In addition to the above experiments, work was done at some length 
with homogeneous light, with gratings of different constants, etc., 
which cannot be detailed here. The most interesting result was obtained 
with a wide slit and white light. It was shown that the fringes are 
ultimately nearly confocal ellipses of enormous eccentricity and with 
the major axis vertical. To produce and center them, the refraction 
(dispersion) of plates is advantageous, if not necessary. 
It is now of interest to turn to the displacement of G' , normal to 
itself, and to consider the resolving power of the system. For the lat- 
ter bears a close analogy to the experiments made in a preceding paper. 
(Carnegie Publication, No. 249, § 37 et. seq., 1916) on the behavior of 
crossed rays. If G' is displaced to G\, over a distance e' = dh (see 
figure 1 where h is the distance apart of G and GO, the rays X' meeting 
in T will now be in the same condition as were originally the rays X. 
In other words, e\ and e\ have become coincident at G\ If we assume 
that rotationally the same type of fringe results in these cases, and if 
V — \ = d\, 6 — 6' = dd, (for the passage of hh' into dd' is in the 
direction from red to violet) 
d 6 = dh sin d cos d/h, nearly. 
Since \ = D sin 6 and d \ = — D cos d d B, this may be changed to 
d X/X = dh (1 - \yD'')/h 
when D is the grating constant. ■ 
The present method, apart from any practical outcome, is worth 
pursuing because of the data it will furnish of the width of the strip of 
spectrum carrying interference fringes, under any given conditions. 
For here the spectra are not reversed or inverted and the latitude of 
interference or diffraction throughout X is much broader than in case 
of reversed spectra. But for this purpose films will not suffice and rigid 
refracting systems must be devised, and the grating constants must 
be quite identical. 
* This article is a note from a Report to the Carnegie Institution of Washington. 
2 The equation is also true for oblique incidence. But for this and the use of homo- 
geneous light with a wide slit, the availability of gratings of different constants, etc., the 
report mentioned may be consulted. 
