The optician to whose talent in his art this fine instrument is due, has recently executed several small telescopes upon the same model, at such a price as to bring them within the reach of amateurs with slender purses. The principal part of these telescopes, one of which is represented in [fig. 49], (see next page), is the mirror, which is about 4 inches in diameter, and 24 inches’ focal length. The body, which is cylindrical, is made of brass, and revolves on two pivots placed horizontally at about one-third of its length from the bottom. The bearings on which the pivots move consist of two upright standards of metal, which are connected at the bottom, and revolve on a pin in the middle of the plate of the tripod stand. They are made of such a height that the lower portion of the instrument may pass between them, when it is necessary to observe objects in the zenith. By the turn of a screw the whole of the upper portion of the instrument may be dismounted and fixed on a lower standard, so that the observer may work sitting down if necessary. The body of the telescope is provided with a finder. One of the great advantages of this form of instrument is that it can be used for observations on the zenith without giving the observer those unpleasant cricks in the neck so inseparable from the use of ordinary telescopes in a nearly upright condition. The mirror will bear a power of 220 diameters, and shows the mountains of the moon, the phases of Mercury and Venus, Saturn and his ring, Jupiter and his satellites, and a large number of double stars and nebulæ. It is provided with a set of eye-pieces, so that any power almost from 50 to 220 diameters may be used at will. The figure on the opposite page will give the amateur a good idea of the form and size of this instrument.
Fig. 49.—Foucault’s Small Telescope.
PART III.
NATURAL MAGIC.
CHAPTER I.
THE MAGIC LANTERN.
The illusions of which we have spoken in the first part of this work depended principally on the nature of man’s vision, who, we found, was the constant and heedless victim of his own powers of sight. We shall now examine a series of illusions that are still more extraordinary, but which have nothing to do with the deceptions practised on us by our visual organs. Instead of being deceived by ourselves, we shall find that we are led astray by others whose knowledge of the laws of optics is greater than our own, enabling them to construct instruments capable of amusing us or imposing on us, according to our ignorance of natural laws. Let us hope, however, that the science of optics has now become so familiar to most educated people, that no such thing as a real imposition can take place, although at the present day there are so many exhibitions of the marvellous that ordinary observers have the greatest difficulty in accounting for them. In former ages, when the knowledge of science was confined to a certain class, the commonest optical facts of the present day were taken advantage of to delude the ignorant. The deceptions practised by the ancient priests of Egypt, Greece, and Rome were undoubtedly many of them of this description. It is a well known fact that both plane and concave metallic mirrors were used by the ancients, and a passage in Pliny gives an account of certain glass mirrors that were made at Sidon. Aulus Gellius, quoting Varro, speaks of the reflecting properties of hollow mirrors, and we shall see, as we go on, what a number of illusions may be practised by means of a series of plane mirrors arranged in a particular way. But we will first devote a short time to the curious historical facts connected with the principle of the magic lantern which took place long before the modern invention of this instrument by Father Kircher.
Brewster says, when treating of this subject, that there can be little doubt that the concave mirror was the principal instrument used in connexion with the pretended apparitions of the gods and goddesses in the ancient temples. In the meagre history of these apparitions that has come down to us, we can easily perceive the traces of an optical illusion. In the ancient temple of Hercules at Tyre, there existed a certain seat made of consecrated stone, out of which the gods rose, apparently at the will of the priests. Æsculapius appeared frequently to his worshippers in his temple in Tarsus, and the temple of Eugenium was famous for the number of gods and goddesses which were constantly visiting its sacred precincts. Iamblicus tells us that the priests showed the gods to the people in the midst of smoke; and when the great magician Marinus terrified his auditory by suddenly showing them the statue of Hercules in the midst of a cloud of incense, it was undoubtedly a woman who performed the part, dressed up in man’s robes for the occasion.