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

176 THE BOOK OF THE LANTERN. figures, and he made many experiments in this direction. He found that almost all the common oils and fats give natural diagrams by which they can be identified; and further, that these figures will vary, according to the length of time for which the oil has been exposed to the air. To get some idea of the nature of these beautiful figures, a drop of pure sperm oil may be allowed to fall on the surface of a pan of water. It will be seen in a few seconds that the film of oil will break up into a number of little open- ings, and that it will exhibit a pattern of great beauty. Rape oil, Lucca oil, and some others, give patterns of en- tirely different designs ; some of them very much resembling beautiful crochet-work. In order to show these patterns in the lantern, we have two or three different methods of going to work. We can exhibit these cohesion figures, for instance, by the simple aid of two pieces of clear glass. Between two such plates put a little vaseline, which in order to increase the effect on the screen may be coloured red with alkanet root. The plates are pressed together, with the vaseline between them, and are then secured by a ring of india-rubber at each end. They are then put into the lan- tern, and while standing upon the lantern stage the blade of a knife is inserted between the two glasses and gradually turned so that they are slightly separated. The effect upon the screen is very beautiful, the disc appearing to be covered with arborescent figures. This experiment may be repeated more than once, but the vaseline will require renewal after a time. Another mode, and perhaps a better one, of showing the same phenomena, is by means of the vertical attachment to the lantern. In this case the