he will make use of that telescope of the mind--speculation, and tell
us much of what his ever-widening researches have led him to conclude
concerning magnetism; a science on which he believes we are shortly to
get large 'increments of knowledge.' Mr Wheatstone, too, having
produced a paper resuming his stereoscopic investigations, had the
honour of reading it before the Royal Society as their Bakerian
Lecture, as I prognosticated a month or two since. Of course in this
practical age the inquiry is put--Of what use is the stereoscope or
pseudoscope? With respect to the former, it is said that artists will
find it very serviceable in copying statuary groups; and a suggestion
has already been made, to adapt it to the purposes of microscopic
observation, as the objects examined will be seen much more accurately
under the extraordinary relief produced by the stereoscope, than by
the ordinary method. And it may interest astronomers to know, that Mr
Wheatstone believes it possible, by means of the same instrument, to
perfect our knowledge of the moon's surface and structure. For
instance: he proposes to take a photographic image of the moon, at one
of the periods of her libration, and a second one about fifteen months
afterwards, at the next libration, which, as you know, would be in the
opposite direction to the first. The two images being then viewed in a
stereoscope, would appear as a solid sphere, in which condition we
should doubtless get such an acquaintance with the surface of our
satellite as can be obtained by no other means. The reason for taking
the images with so long an interval between is, that although each one
represents the same object, each must be taken at a different angle;
and for an object so distant as the moon, the difference caused by the
libration would, it is believed, be sufficient for the desired result.
In the small pictures, however, the difference of angle is so slight,
that to the unpractised observer they appear precisely alike; it is,
nevertheless, essential to the effect that the variation, though