The book of lantern ; being a practical guide to the working of the optical (1888)

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THE BOOK OF THE LANTERN. 181 S. Pichler, who designed various other ingenious contriv- ances. He was very jealous about this astrometeoroscope, and the onlj one made was at the Polytechnic Institution, where it was carefully kept under lock and key, except when in actual use. When the apparatus of the institution came to the hammer, I remember that there was some little excitement when the astrometeoroscope was put up for sale. Opticians and others would have been glad to get hold of it, so as to have multiplied it for sale. This led to a brisk competition, ending with Mr. Pichler giving an extravagant price for his own bantling. And in that way the secret remains in the hands of a few only, and perhaps it would be unkind to divulge it. But, at any rate, I cannot do much harm by giving a general idea of the out- ward appearance of the instrument and its capabilities. The astrometeoroscope consists of a narrow box thirteen inches in length, and of such a width that at one end it will fit the stage of the lantern. At this end it has the usual three-inch disc opening, which is occupied by two plates of metal which are scored across obliquely with slits and which are superposed one. on the other, so that the slits on each cross one another diagonally. Now it is clear that the only places where light can pierce these plates of metal so as to make itself evident on the screen is in those places where the slits on the plates intersect one another. The effect on the screen, therefore, whilst the instrument is quiescent, is a series of dots of light all over the screen, but at regular distances from one another. By very ingenious mechanism the two plates are caused to move to and fro in contrary directions, and the