THE BOOK OF THE LANTERN. 27 



in sack a case terribly impede the view of all those 

 behind the lantern, besides causing much disarrange- 

 ment of chairs. But let him use an 8-inch objective, 

 and his lantern can he carried to a distance of 

 35 feet from the screen. This will probably place the 

 lantern quite at the back of the hall. Another point in 

 favour of the latter position is, that if his two lenses are of 

 the same diameter, as most probably they will be, the long, 

 focus lens will admit more light, as already explained ; 

 while it will certainly give better definition and less dis- 

 tortion jbhan the shorter focus lens. One more circum- 

 stance in favour of the longer focus lens is, that the lantern 

 is kept more horizontal. When the instrument is close to 

 the screen, unless the floor of the auditorium be inclined, 

 as in a proper lecture-theatre, the lantern must be very 

 much tipped up at its fore-end, so that the disc on the sheet 

 shall be high enough. This raising of the. lantern, of 

 course, leads to distortion, unless the screen be inclined 

 towards the lens so as to compromise the matter. With a 

 long-focus lens the distortion from this cause is greatly 

 reduced, and very often it is so slight that there is no need 

 to incline the sheet. I hold the opinion that a good single 

 achromatic lens of long focus is by no means to be despised 

 for lantern work, although a half-plate lens of the portrait 

 type is to be preferred. Such a lens can only be used for 

 the long-distance work. If it is absolutely necessary that 

 the lantern be as near the sheet as possible, then it is com- 

 monly the fashion to use a French quarter-plate photographic 

 lens, I presume on account of its cheapness. For it is 

 most difficult to select a lens of this description which will 



