PHYSICS: C. BARUS 
379 
per sing power above a certain minimum. Thus in a grating of less 
than 7,000 lines to the inch, the undeviated slit image and the image of 
the first order are not clearly resolved, unless the slit is ver}' fine. In 
the second and higher orders, however, the resolution is very pronounced 
and the fringes stationary. 
The resolution of fringes is equally manifest in front of, oj behind the 
principal focal plane, so that if a weak convex lens is added to the 
objective of the telescope, the succession of fringes is found with an out- 
going ocular; if a weak concave lens is added to the objective, with an 
ingoing ocular, starting in each case near the principal f^cus. As the 
fringes increase in size they in turn subdivide, as if each fringe were 
a new slit image, capable of undergoing secondary diffracdon. Beyond 
these secondary fringes no further resolution was detectec^. 
Returning to the work with two successive gratings aad white light, 
the channelled spectra obtained are too complicated for concise de- 
scription. A very interesting result, however, is the passage of the 
fringes across the stationary sodium line, when the grating is moved, 
fore and aft, in a direction normal to its plane. The region of the D 
line is thus alternately dark and bright. The direction of these rays 
remains unaltered while the illumined strip is shifted horizontally across 
the ruled space of the second grating. It is sometimes difficult to see 
the D line in the focal plane of the fringes. When homogeneous light 
is used this fiducial mark is necessarily absent and the cross hairs of the 
ocular must be supposed to replace it. The shift of the fringes is then 
equally obvious and sometimes (sodium light) different groups seem to 
travel in opposite directions while the grating moves in one direction. 
In case of homogeneous light and two gratings, moreover, the fringes 
seem to be of minimum size in the conjugate focal plane of the gratings. 
They increase in size and in turn split up, in focal planes before and 
behind this. 
An insight into these occurrences was finally obtained in observation 
with homogeneous light, on the spectrometer, by shifting the grating 
(transmitting) in its own plane, right and left. The fringes in such a 
case move bodily across the field of the telescope, new groups entering 
on one side for those which leave on the other. These fringes, even if 
quite distinct, are differently arranged in the coarse and fine series and 
are frequently accompanied by dark or bright bands. If the ocular is 
drawn out and set outward from the principal focal plane (at which the 
slit image is quite sharp) into a different position, the fringes move in a 
direction opposite to the grating. If the ocular is set inward from the 
principal focal plane, they move in the same direction as the grating. 
