THE BOOK OF THE LANTERN. 



21 



consists of a pair of plano-convex glasses mounted in one 

 cell, with their curved surfaces all but touching one another. 

 This form was, I believe, first introduced with the 

 American sciopticon. It is shown at fig. 12. The other 

 form of condenser is that devised (but not for lantern use) 

 by Sir John Herschell, and which consists of a double 



FIG. 12. 



FIG. 13. 



convex lens, associated with a meniscus, the concave side 

 of the latter being next the radiant point, as shown in 

 fig. 13. In a good condenser we want not only quantity 

 of light, but also good quality, and these properties can 

 only be secured by careful attention to certain points of 

 construction. Quantity of light is governed by size and 

 focal length, as already pointed out, and it may be as well 

 to indicate here the manner in which lanterns furnished 

 with condensers of larger size than I have recommended 

 may be made to transmit a greater amount of light by the 

 interposition of another lens. There were at the old Poly- 

 technic Institution some antique lanterns with 10-in. con- 

 densers, this large size being necessary to cover the 8 -in. 

 hand-painted transparencies which were in use before 

 photography worked a revolution in such things. Such a 

 large condenser, of course, meant a great loss of light, as 



