comparing stereoscopically two star photographs taken at different
dates. Wolf of Heidelberg has used this for many purposes. His
investigations depending on the solar motion in space are remarkable.
He photographs stars in a direction at right angles to the line of the
sun's motion. He has taken photographs of the same region fourteen
years apart, the two positions of his camera being at the two ends of
a base-line over 5,000,000,000 miles apart, or fifty-six astronomical
units. On examining these stereoscopically, some of the stars rise out
of the general plane of the stars, and seem to be much nearer. Many of
the stars are thus seen to be suspended in space at different
distances corresponding exactly to their real distances from our solar
system, except when their proper motion interferes. The effect is most
striking; the accuracy of measurement exceeds that of any other method
of measuring such displacements, and it seems that with a long
interval of time the advantage of the method increases.

_Double Stars._--The large class of double stars has always been much
studied by amateurs, partly for their beauty and colour, and partly as
a test for telescopic definition. Among the many unexplained stellar
problems there is one noticed in double stars that is thought by some
to be likely to throw light on stellar evolution. It is this: There
are many instances where one star of the pair is comparatively faint,
and the two stars are contrasted in colour; and in every single case
the general colour of the faint companion is invariably to be classed
with colours more near to the blue end of the spectrum than that of
the principal star.

_Binary Stars._--Sir William Herschel began his observations of double
stars in the hope of discovering an annual parallax of the stars. In
this he was following a suggestion of Galileo's. The presumption is