Directions to be observed for the Attention of the Lamps. The Projection Screen. 



155 



50 729 E. 



>> 



a 

 a 



| 



5 



A 

 O 



a! 



Phenomenon Xo. 1 indicates: Source of light too far to the right; 



No. L> .. left: 



high: 



,, .. low: 



far from condenser: 



,, , close to condenser; 



No. 3 

 Xo. 4 



\o. ."> 



No. 6 



No. 7 



,, the Source of light is correct. 



In the projector lanterns with arc lamp suspended vertically, the latter is so arranged that 

 mostly only Xos. 3 and 4 of the above phenomena are likely to occur. The arc lamp is so adjusted in 

 its position that it always casts its light on to the condenser in the correct manner. The height of the 

 lamp cannot therefore be adjusted once and for all at the commencement, as (see Fig. 50,728 C) 

 exactly the same angle is not always encountered when the carbons are being adjusted and, therefore, 

 the light pencil may on one occasion be cast higher than on another. This necessary slight adjustment 

 is completed easily and quickly in the Model B projectors by the aid of the handle and screw, S 

 (Fig. 50,728 B) placed on top of the lantern. 



Attention required with other Sources of Light. When employing limelight the lime cylinder 

 or disc should be first heated by house gas at the ordinary pressure, and when this part of the burner 

 is sufficiently hot, the stopcock of the oxygen cylinder should be opened and the pressure regulated 

 by means of the pressure-reduction valve until the maximum brilliancy is obtained, after which the 

 apparatus should be allowed to continue burning. 



The same adjustment when using the apparatus is necessary in the case of thorium discs, which 

 are now used instead of zirconium on account of the much better light they give and their greater du- 

 rability 



The adjustment of other systems of illumination is also provided for at the commencement, 

 so that scarcely any alterations whatever are necessary to the original arrangement. As regards the 

 attention necessary to the individual lamps, we supply, in such cases where it appears necessary, spe- 

 cial instructions when supplying the lamps. When inserting lamps in projection lanterns not built 

 up by us. the directions, given in the case of the electric lanterns should be observed. It is 

 advisable in such cases, however, to send lantern and lamp to our works in order that they may 

 be made to fit properly. 



The Projection Screen. 



The projection screen is in the generality of cases fixed to the ceiling in front of the students, 

 on the blackboard wall, and in front of the blackboard. For preventing it from becoming dusty it is kept 

 rolled up. It is specially advantageous to place the screen on the blackboard wall in cases in which 

 the lantern is erected in front of the students or in their midst, as the apparatus set up on the lamp 

 can be seen well from the individual places. In this respect, however, it is also of value to arrange the 

 screen on one of the lateral walls of the class room, opposite the window wall and the heliostat in the 

 prolongation of the lecture table, as the projector screen can then also be used for the experiments 

 with the heliostat. 



The height at which the screen should be hung is determined by carefully considering the place 

 where the lantern is set up and the correct position of the same from the screen and seeing that no^sha- 

 dows of either forms or scholars appear on the lower part of the screen. In many instances, when the 



Cl. 226. 



