CH. V] MAGIC LANTERN WITH PETROLEUM LAMP 121 



flame or incandescent mantle) , may be utilized in making the screen 

 image. This is of fundamental importance (fig. 64, 90). 



(E) Use of twilight vision. It is astonishing how dim a picture 

 can be clearly seen after one's twilight vision has become fully 

 established. According to careful investigations the sensitiveness 

 of the eye may be increased from 35 to 2500 times by the adapta- 

 tion to dim light ( 281). 



The old lanternists used to advise that the exhibition should not 

 begin until the audience had been in the darkened room for half an 

 hour "to get," as they said, "the sunlight out of their eyes." We 

 would say to "get the twilight vision well established." 



194. Time required for lighting up. The gas light and the 

 acetylene light are quickly established, but the petroleum and 

 the alcohol lights require several minutes to get up the best illumi- 

 nation. These two should then burn during the entire time of an 

 exhibition. If the lecturer cannot arrange to have all the slides 

 continuously, but must have them at intervals during the lecture, 

 the operator should make use of an objective shield (fig. 14, 62), 

 and leave the lights on all the time. 



195. Rehearsals. As these lights are more difficult to man- 

 age and the results are less satisfactory than with the more power- 

 ful radiants, so much the more should the operator rehearse before 

 the lecture and make sure that everything is in as nearly perfect 

 order as human skill can make it. 



THE MAGIC LANTERN WITH A PETROLEUM LAMP 



196. The petroleum lamps now used as radiants for projec- 

 tion have two, three or four wicks. The wicks are wide (about five 

 cm., two in.) and are placed edgewise to the condenser. If more 

 than two wicks are used the two outer ones are inclined inward 

 (%. 66). 



Sometimes instead of being ranked side by side, the different 

 wicks are arranged like the lines forming the letter W, but there is 

 no advantage in this. 



