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THE BOOK OF THE LANTERN. 177 lantern slide must take the form of a shallow box, having a glass bottom. Such a box can easily be made by fit- ting a piece of thin glass, say, 3J inches square, into a frame of wood half an inch in height, and cementing the glass in a groove with marine glue. The glass cell so provided should be placed in a horizontal position upon the stage, and be filled with water. Dif- ferent oils can then be dropped on to the surface of the water, and the characteristic cohesion figures due to each will be thrown upon the screen. If this latter mode of showing the phenomena be chosen, it is obvious that a different glass cell must be used for each oil exhibited, and I think that it would be quite possible to produce lantern slides direct from these oily cohesion figures ; although I have not experimented in this direction myself. The principle employed would be that of " Lithography." The oily figures might be transferred to a piece of glass direct from the surface of the water. Those figures could be darkened to any extent by employing a greasy printing ink, taking care to wet the glass so as to repel the ink; but this is a matter into which,! cannot now afford space to enter, and I merely allude to it as a field for profitable experiment. Many pieces of apparatus have been devised for the lantern, which exhibit the principle of what is known as kt persistence of vision." In order that we may thoroughly understand in what this principle consists, I may men- tion that the human eye possesses a peculiar property which is highly convenient to its proprietor. What is meant by " persistence " is that the retina has the power N