THE PARTS OF A LANTERN 29 



These photographic lenses have however the advantage, that 

 very often by unscrewing the double lens at the back, and 

 substituting for it the front lens reversed, or with its convex 

 side to the slide, a very good long-focus single lens is obtained 

 of the (a) kind. 



The best lantern-makers have lately, taking this photo- 

 graphic lens as a basis, worked out by screen tests improved 

 curves for lantern use only. Such are sent out in the highest 

 class lanterns. These lenses reverse the two single lenses F, c, 

 of the Petzval system (fig. 18), need no stop whatever, and give 



FIG. 18 Double Combination 



magnificent definition ; but, as a rule, they are not corrected 

 for photography, and the single lens can rarely be used satis- 

 factorily. It only remains to combine these points with the 

 other excellent qualities already attained ; and there is little 

 doubt that the growing use of enlarging lanterns, or adapta- 

 tion of the lantern to photography direct, with the aid of the new 

 optical glasses now made in Germany, will enable this crown- 

 ing perfection to be obtained, at least in all lenses of 6-inches 

 focus and upwards. With lenses of 4^ inches focus, used 

 chiefly for paraffin oil lanterns in small rooms, to exhibit 

 through a transparent sheet, a perfect image of a slide 3 inches 



