POOJECTIONS. 



27 



jected, the smaller must be the lens used as the object- 

 ive, and the shorter must be its focal length ; hence, if 

 a beam of parallel rays is used, it may often be so small 

 as to be nearly useless, for the divergence is so rapid 

 beyond the focus of a short focus lens that the little 

 light thus used would be too much scattered. It be- 

 comes necessary, then, to make a large beam of light 

 to pass through the small lens. This is accomplished 

 by means of a second lens, called the condenser, because 

 its office is to condense a large number of light rays for 

 the double purpose of illuminating the object better 

 and making them all to pass through the smaller lens. 

 This condenser is usually four or five inches in diameter, 

 though for special purposes it is sometimes made a foot 

 or more in diameter. For the porte lumiere the con- 

 denser may be the same lens that was used as the ob- 

 jective, or any lens may be used that has a sufficient di- 

 ameter and a focus of from one to two feet. It may be 

 either double-convex or plano-convex. 



TO PROJECT WITH A CONDENSER. 



The object d (Fig. 18) is placed near the condenser 

 f, and the objective o is brought near to it and slowly 



moved toward 

 the screen, as 

 before, until 

 the well de- 

 fined image ap- 

 pears upon it. 

 It must be not- 

 Fig. 18. ed here that 



the size and focal length of the objective must be such 

 that all the light passes through it when it is at its 

 proper distance from the object. If be moved toward 



