150 THE ART OF PROJECTING. 



lens, and with a less powerful one pieces of wood and 

 paper may be ignited. 



A transparent solution of common alum is opaque 

 to the same rays that are so easily transmitted by the 

 iodine solution. 



A test-tube filled with water placed at the focus of 

 the obscure rays in a minute or two may be made to 

 boil ; an air thermometer will scarcely be affected at 

 that place. 



MAGNETISM. 



With the vertical attachment to the lantern the 

 phenomena of magnetism are best shown. 



1. Have two or three small magnetic needles 

 mounted upon needle points thrust through pieces of 

 cork, so as to turn freely. Place one upon the upper 

 face of the condenser to the vertical attachment, and 

 project it sharply upon the screen. A piece of iron or 

 another magnet brought into its neighborhood will 

 disturb it, and every motion will be plainly noticeable 

 as well as the direction of the exciting body. 



2. Place two of these needles near to each other, 

 but not so near as to touch, and give to one of them a 

 twirl so that it revolves upon its support. It will soon 

 set the other revolving and it may be stopped itself 

 after setting the second one going, and afterward be 

 again started while the other one stops. 



3. Place a third, quite small one not more than 

 half an inch long in the neighborhood of the other 

 two, and again set the one whirling. 



4. The magnetic phantom. 



Lay a small magnet an inch or two long upon the 

 upper condenser ; and upon the magnet lay a piece of 

 clear glass three or four inches square. Project the 

 magnet, and then scatter from a small sieve, or gently 



