378 
PHYSICS: C. BARUS 
well-known ellipsoidal axis and the solar velocity increasing as we go 
from the stars of small and 'contrary' proper motion to those of 
medium and large proper motion. 
Such variations of the position of the apex although apparently 
connected with distance or other conditions, would seem to point 
ultimately to some form of rotary or spiral motion among the stars 
themselves. 
The details of the investigation, which is based upon Campbell's 
well-known catalogs of about 1300 radial velocities, will be published 
in the Astro physical Journal. 
CHANNELED GRATING SPECTRA, OBTAINED IN SUCCESSIVE 
DIFFRACTIONS 
By C. Barus 
DEPARTMENT OF PHYSICS. BROWN UNIVERSITY 
Received by the Academy, June 13, 1916 
The great variety of channelled spectra obtained, when white light 
is successively diffracted by two gratings, are referrable to the fringes 
of the diffraction of homogeneous light, observed outside the principal 
focal plane, on a spectrometer. In other words, if light of a given 
pure color (sodium, mercury) is used, a single grating suffices. Each 
line of the spectrum is resolved into well defined groups of fringes, if 
it is observed either in front of or behind the principal focal plane. The 
arrangement of fringes varies in marked degree with the distance of the 
plane observed, from the latter. If reflecting gratings are used, there 
is no other possible source of interferences; but reflecting and trans- 
mitting gratings show the phenomenon equally well. 
After finding how easily the Fresnellian interferences of two virtual 
slits could be reproduced in the telescope and observed on either side 
of (before or behind) the plane of the sharp sHt images, it seemed reason- 
able to suppose that the diffraction of a slit could also be produced and 
exhibited in this way; but the availability of this anticipation is attended 
with much greater difficulty. The image of a very distant sHt does 
indeed show separated diffraction fringes on either side of the principal 
focal plane in the observing telescope. But they move right and left 
with the eye, in the same direction and in this respect do not at once 
recall the phenomena under consideration. Usually the blurred image, 
out of focus, is stringy, without definite structure. 
To obtain sharp stationary fringes from an image of the slit, this 
image must be produced by the diffraction of a grating, having a dis- 
