THE PARTS OF A LANTERN 19 



The condenser at E, consisting of two plano-convex lenses, 

 was also used by Herschel, was adopted by Mr. Marcy for 

 his Sciopticon lamp, and is the best form for all large and 

 hot radiants. Such lamps have to be placed at a greater 

 distance from the condensers to avoid cracking them, and are 

 generally used with short focus objectives ; the two cones of 

 light being thus more alike in angle, two such lenses represent 

 pretty well the optical conditions. When properly modified, 

 this form is equally adapted for the lime-light, and is now the 

 most usual condenser found in good lanterns. 



10. Correction of the Condenser. For oil lanterns the 

 plain form shown at E, of two similar lenses, cannot be 

 improved. Optically, it would be better that the lens next 

 the lamp should be rather deeper in curve, but the great heat 

 makes this undesirable. With the lime-light it is different, 

 and in considering the very best form, it is well to understand 

 the principles upon which the correction of aberrations 

 depends. This has been very familiarly explained in the 

 diagram, fig. 11, by Mr. 

 Eobert Bow. Here D E re- 

 presents the upper half of 

 a plano-convex lens, the 

 faint line h e the outline 

 of a double convex lens in 

 contact with it, and de a 

 meniscus lens of the same 

 focus. Consider now the 

 different effect of these two 



latter upon the spherical aberrations, due to the fact that 

 the marginal rays are brought to a shorter or closer focus 

 than the central rays. It is quite plain that the focus F of 

 the central rays, A A, will be almost exactly the same in 

 the two arrangements we may practically consider that a 

 fixed point. But with the marginal rays, B B, it is different. 

 Tracing the top one, B D, let us suppose that the lens h re- 



02 



