26 THE ART OF PROJECTING. 



the lens will cost but a trifle. The pictures themselves, 

 though not half as costly as they were before photogra- 

 phy was applied in making them, can be rented of any 

 one who keeps them for sale, if one cannot afford to 

 buy them outright. Most excellent transparencies, on 

 all sorts of subjects, can be bought, from six to nine 

 dollars a dozen, of any lantern-maker or dealer in pho- 

 tographic materials. 



If the teacher wished to give a lesson on the elements 

 of drawing, his copies could be prepared upon glass, by 

 one of the methods given a little further on. These, when 

 projected, would be so large that a large school could see 

 them as plainly as if they had been drawn upon a huge 

 blackboard, with chalk. The room could be light 

 enough for any of the required work. Geometrical fig- 

 ures, outline maps, botanical specimens, the kaleido- 

 scope, chemical reactions in a large test tube ; natural 

 history specimens, such as small fish, pollywogs, water 

 beetles, butterflies, grasshoppers ; the splendid colors 

 on huge soap-bubbles ; the vibrations of tuning-forks, 

 and of cords ; the intensity of light, reflection, refrac- 

 tion, magnifying power of lenses, and many more things, 

 may be projected, in an admirable way, with only these 

 two pieces. 



THE CONDENSER AND ITS USE. 



The rays of light reflected from the mirror a (Fig. 17) 

 through the aperture, are parallel, and the diameter of 

 the lens o should be as great as the thickness of the 

 beam, and it may have a greater diameter with advan- 

 tage. If it has less, some of the light will pass its 

 edge, and either be unused or, what would be worse, 

 fall upon the screen and make a bright spot in the mid- 

 dle of the picture. The smaller the object to be pro- 



