but he desired no apparatus that permitted of
reconstituting the relief on looking at them. Those savants who, after
him, occupied themselves with the question, treated it no further
than from a theoretical point of view. It was not till 1838 that the
English physicist Wheatstone constructed the first stereoscopic
apparatus permitting of seeing the relief on examining simultaneously
with each of the eyes two different images of an object, one having
the perspective that the right eye perceives, and the other that the
left eye perceives.

This apparatus is described in almost all treatises on physics. We may
merely recall the fact that it operated by reflection, that is to say,
the two images were seen through the intermedium of two mirrors making
an angle of 45 degrees. The instrument was very cumbersome and not
very practical. Another English physicist, David Brewster, in 1844
devised the stereoscope that we all know; but, what is a curious
thing, he could not succeed in having it constructed in England, where
it was not at first appreciated. It was not till 1850 that he brought
it to Paris, where it was constructed by Mr. Soleil and his son-in-law
Duboscq. Abbot Moigno and the two celebrated opticians succeeded, not
without some difficulty, in having it examined by the _official_
savants; but, at the great exposition of 1851, it was remarked by the
Queen of England, and from this moment Messrs. Soleil & Duboscq
succeeded with difficulty only in satisfying the numerous orders that
came from all parts. As photography permitted of easily making
identical images, but with different perspective, it contributed
greatly to the dissemination of the apparatus.

The stereoscope, such as we know it, presents the inconvenience of
being incapable of being used by but one person at once. Several
inventors have endeavored to render the stereoscopic images visible to
several spectators at the same time. In 1858, Mr. Claudet conceived
the idea of projecting the two stereoscopic images upon ground glass
in superposing them. The relief was seen, it appears, but we cannot
very well explain why; the idea, however, had no outcome, because the
image, being quite small, could be observed by but three or four
persons at once. It was Mr. D'Almeida, a French physicist, who toward
the same epoch solved the problem in a most admirable manner, and we
cannot explain why his process (that required no special apparatus)
fell into the desuetude fro