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 



