H2 LIME LIGHT WITH OXYGEN AND GAS [Cn. IV 



If everything has been put in perfect order before the exhibition 

 the changes required during the exhibition should be very slight. 



171. Arrangement of lantern slides, their insertion and 

 focusing. Follow the directions in Ch. I, 21-23; 35~4 I - 



172. Lighting the room. As the lime light gives only about 

 Vs to VG as much light as the arc lamp the room must be darker 

 if the same brilliant contrast is desired. One can determine by a 

 little experiment with the set of slides to be exhibited at any time 

 how dark to have the room. The more transparent the lantern 

 slides, the lighter can the room be. Many lantern slides are 

 altogether too opaque, and require a dark room, no matter what 

 light is used in the lantern. 



173. Avoidance of intervals of total darkness in the room. 



This can be accomplished by leaving the lantern on all the time and 

 by using the objective shield (fig. 62). If that device is not used, 

 then the operator should not turn out the lime light until the room 

 lights are turned on. And whenever the lantern is to be used, the 

 lecturer must give two or three minutes warning to the operator 

 before turning off the room lights. 



THE LIME LIGHT WITH OXYGEN AND ILLUMINATING GAS 



174. Frequently the lime light is produced with illuminating 

 gas drawn from the house supply, and with oxygen gas in a steel 

 cylinder ( 154). 



If illuminating gas is used instead of hydrogen it is to be remem- 

 bered that the pressure as drawn from the house supply is very 

 slight, i. e., about equal to a column of water from 5 to 12 cm. 

 high (2 to 5 in.) or only about Vr, the pressure of the hydrogen 

 and oxygen when these gases are drawn from steel cylinders ( 1 54). 



The oxygen is used at a much higher pressure than the house gas, 

 and many operators use for this combination the "blow-through 

 jet" (fig. 58). Mixed jets arc also constructed for this combina- 

 tion, but the "blow-through" is considered safer. The user of this 

 form of apparatus would do well to get the combination found best 

 by the manufacturers of his apparatus. 



