270 



THE BOOK OF THE LANTERN. 



He again saw the error, took it out of the lantern, and re- 

 placed it, but on its other side ; so, in reality, this genius 

 tried every conceivable way of showing 

 that picture but the correct one. 



It is imperative that there should be 

 some well-understood code of signals be- 

 tween the lecturer and the lantern operator, 

 for in many cases they are 50 feet apart. 



Some lecturers are content with verbal 

 directions, but these are simply intoler- 

 able to any one with any idea of what 

 a lecture should be. To hear a man 

 calling out, " Next picture, please," and 

 so on, utterly spoils even a lecture which 

 is good in other respects. An audible 

 sound signal, such as tapping with a 

 pointer, or sounding a gong when the 

 picture is required to be changed is 

 almost as bad. A lecture lamp has 

 recently been introduced, which not only 

 comprises a shade light for the lecturer's 

 desk, but has at the back, i.e., the side 

 presented towards the audience, and there- 

 fore towards the lantern, a little disc of 

 red glass, which is uncovered by touching 

 a lever at the side of the lamp. See S, 

 fig. 74. When the operator sees the little red flame dis- 

 closed he knows that a fresh picture is wanted, and should 

 he be inattentive to that signal there is a little gong, B, below 

 the lamp which can be used on such an emergency. This 



FIG. 74. 



