THE BOOK OF THE LANTERN. 2i> 



perfect lenses for use with the instrument. Mr. Dallmeyer 

 thereupon determined to tackle the subject, and produced 

 new forms of condensers and objectives, a description of 

 which he placed before the Photographic Society. I can- 

 not do better than quote the description of the new eon- 

 denser from this paper, and at the same time reproduce 

 one of the drawings shown in illustration of the remarks 

 made (see fig. 15) : 



FIG. 15. 



" The condenser is of 4-in. effective diameter, and 2J- 

 in. equivalent focal length. Assuming the light to be at 

 a safe distance of 2| in. from the flat surface of the first 

 lens, this condenser collects an angular pencil of about 

 66, i.e., about 20 per cent, more light than the shortest 

 focus symmetrical. It consists of two unsymmetrical 

 lenses, A and B. A is a plano-convex of flint 3|-in. 

 diameter, and B is a double convex of crown glass of 4-in. 

 diameter. The lenses are mounted at a certain distance 

 apart, with their deep sides facing each other. Approxi- 

 mate correction of chromatic aberration for centrical pencils 

 is obtained by a proper apportioning of their focal lengths, 

 and the distance at which they are placed. Thus : ray 



