OPTICAL PROJECTION 



bottom. The top roller of the screen itself keeps them the 

 proper width apart. 



A tracing-paper screen IB always furnished with a portable 

 stand, so constructed that if necessary it can be placed upon 

 a table. 



Transparent calico screens are usually stretched across 

 the space occupied by folding doors. This is easily done by 

 lacing twine lightly to large tacks, or even stout drawing-pins, 

 which can be removed without leaving any conspicuous sign. 



71. Light for the Lecturer. Most of the opticians supply 

 a sort of fold-up desk, which shades the light from all but the 

 copy of the lecture ; but such apparatus is now out of date, 



being superseded by a simple lamp. If 

 this is properly shaded, somewhat as in 

 fig. 71, which is a pattern very widely 

 adopted in general, with little variations 

 in detail, all necessary purposes will be 

 equally secured, and some sort of desk 

 or table can always be procured. These 

 lamps can be had with a red glass signal 

 to the operator at the lantern, who 

 watches for this direction when to change 

 the slide ; and at option with a signal- 

 bell also. A bell, however, is a nuisance 

 in a lecture. Of late the excellence and 

 cheapness of electric bell apparatus has introduced the fashion 

 of carrying a wire from lecturer to operator, which operates 

 a very small signal bell for him alone ; but even this is audible 

 unless the lantern is worked from a box or gallery. A signal 

 by red light is the best. Any signal should be given some 

 seconds before the change is required. Of course any operator 

 acquainted with the lecture knows when to change without 

 'any signalling at all. 



72. Stand for the Lantern. It is a great comfort when 

 the lantern can be worked from a gallery or box at the other 



FIG. 71. Lamp 



