174 THE BOOK OF THE LANTERN. 



is connected with its own terminal ; these two latter ter- 

 minals in the cut being lettered bb, and signifying that 

 they should be joined up to the battery employed. The 

 other two, which are lettered gg, are fastened to the 

 terminals on the galvanometer slides. 



In practice it is best for the current reverser to be placed 

 on the lecturer's desk, at some distance from the lantern, 

 while the galvanometer slide is joined up by means of tem- 

 porary wire connexions. The lecturer then has the power 

 of reversing the current by touching with his finger either 

 of the two treadles, and he can demonstrate in the most 

 perfect manner how the different letters in the tele- 

 graphic alphabet are made up of movements of the needle 

 to the right or left, as the case may be. He can also point 

 out that the " dots " and " dashes " of the Morse system 

 correspond with these right and left hand movements of 

 the magnetic needle. 



In the old days of the Polytechnic Institution in Regent 

 Street, which was the resort of so many delighted schoolboys 

 and girls, there were several experiments performed with the 

 lantern which, so far as I know, have not been repeated 

 elsewhere. One of the most curious was the movements of 

 the legs of a frog. This is rather a difficult experiment to 

 perform, but when well done, is highly effective on the 

 screen. 



At the Polytechnic the frog's legs covered the large 

 screen, and were thus magnified to about 26 feet. The legs 

 were hung to a special form of slide, and the nerves and 

 muscles of the dead frog were touched with metallic wires, 

 when they immediately kicked out in the most startling 



