LANTERNS. 15 



All the rays which will fall upon the object will be in- 

 cluded in the angle b a c. Interpose the lens de be- 

 tween the light and the object, and all the rays 

 included in the much greater angle d a e will now 

 fall upon b t, and it will be much brighter. There is 

 always another lens, and ^sometimes two, called the 

 objective, in front of the lantern, to give definition to the 



picture. All the 

 essential parts 

 of a lantern are 

 shown i n the 

 accompanying 

 picture : a is 

 the source of 

 light inclosed in 

 the box b; d e, 

 the lens for di- 

 verting a greater number of light-rays, and called the 

 condenser; c is the objective ; and s, the screen to receive 

 the light. 



The forms of lanterns differ somewhat as they are 

 adapted to different purposes, and they are called 

 stereopticons when the electric, the magnesium, or the 

 lime light is used in them. 



The electric lamp is generally placed within the lan- 

 tern box, which is made to accommodate either it, or 

 the lime light, as is most convenient or desirable. 



The following engraving (Fig. 7) represents the essen- 

 tial conditions for the lime light in a very convenient 

 form. Within the lantern may be seen the fixtures for 

 holding the lime and the jet, with the rubber tubing to 

 connect with the gas tanks. The front side and back 

 are provided with heavy black-cloth curtains which may 



