y spent in such
searches without result. We need only recall Hencke's fifteen years of
fruitless search, before finding a minor planet, to realise this fact.

[Sidenote: Superposition of plates.]

[Sidenote: The stereo-comparator.]

One thing of importance may be done; we may improve our methods of making
the search, so as to economise labour, and several successful attempts
have already been made in this direction. The simplest plan is to
superpose two photographs taken at different dates, so that the stars on
one lie very close to those on the other; then if an image is seen to be
unpaired we _may_ have found a new star, though of course the object may
be merely a planet or a variable. The superposition of the plates may be
either actual or virtual. A beautiful instrument has been devised on the
principle of the stereoscope for examining two plates placed side by side,
one with each eye. We know that in this way two photographs of the same
object from different points of view will appear to coalesce, and at the
same time to give an appearance of solidity to the object or landscape,
portions of which will seem to stand out in front of the background.
Applying this principle to two photographs of stars, what happens is this:
if the stars have all remained in the same positions exactly, the two
pictures will seem to us to coalesce, and the images all to lie on a flat
background; but if in the interval between the exposures of the two plates
one of the stars has appreciably moved or disappeared, it will seem, when
looked at with this instrument, to stand out in front of this background,
and is accordingly detected with comparatively little trouble. This new
instrument, to which the name Stereo-comparator has been given, promises
to be of immense value in dredging the sky for strange bodies in the
future. I am glad to say that a generous friend has kindly presented the
University Observatory at Oxford with one of these beautiful instruments,
which have been constructed by Messrs. Zeiss of Jena after the skilful
designs of Dr. Pulfrich. Whether we shall be able to repeat by deliberate
search the success which mere accident threw in our way remains to be
seen.

CHAPTER V

SCHWABE AND THE SUN-SPOT PERIOD

[Sidenote: Discoveries contrary to expectation.]

In preceding chapters we have reviewed discoveries, some of which have
been made as a result of a deliberate search, and others accidentally in
the course of