THE PARTS OF A LANTERN 17 



warming his slides. If the plate itself is too hot to lay them 

 upon, they can be readily supported on three small slabs of 

 wood or cork. Opticians have long been behindhand in this 

 respect, and such a low flat top has only to be known to be 

 universally approved of. 



In the United States, lanterns are sometimes made with 

 no rigid body at all. One such which was purchased there, 

 and used in this country by the late Mr. E. A. Proctor for his 

 lectures, had for its base a light metal quadrangle supported 

 on four short pillars at the corners. The front alone, carrying 

 the lenses, was hinged to this, and a light body fitted on the 

 base behind like the tilt of a waggon. Another common 

 American plan (shown hereafter in Chapter XII.), is to hinge 

 the front to the base in the same way as just described, to add 

 two light pillars behind, on which and the front is supported 

 a sheet-iron top, and to form the sides of black cloth curtains. 

 The arrangement may occasionally be useful for its lightness, 

 and for packing into a small space ; but so far I have met 

 with none who liked it in this country, in comparison with the 

 more solid English bodies. 



9. The Condenser. The use of this has been explained. 

 A single lens is, however, only used in toy lanterns. The 

 object is to take up as wide an angular pencil of rays from 

 the radiant as possible, and send them through the objective. 

 A single lens, to do this, must be of such thickness as to lose 

 much light by absorption ; and moreover the chromatic and 

 spherical aberration would be excessive. The use of two or 

 more lenses accomplishes the object with a moderate thick- 

 ness of glass, and also allows a large part of the aberrations 

 to be corrected. 



Fig. 10 shows the principal forms of double condensers 

 which have been used. Two double convex lenses, c, were 

 used by Messrs. Carpenter & Westley in the early phantas- 

 magoria lanterns, and for many years afterwards, till very 

 lately, in the electric lanterns of M. Duboscq. They aro 



C 



