CH. XII] TROUBLES WITH ROOMS AND SCREENS 471 



general rule can be given for the size of screen necessary. For 

 large halls the screen used for the magic lantern usually answers. 

 In small rooms for special demonstrations it is advantageous to 

 have a movable screen on a stand that can be varied in distance for 

 different conditions. The magnification and the objective neces- 

 sary for the same must be determined in each case by the lecturer 

 before the lecture or demonstration (see 400, Ch. IX.) 



641 . Troubles with Rooms and Screens : 



1. Poor image on the screen. This may be due to 



(a) Insufficient light from the radiant; 



(b) Too much light in the room; 



(c) A poor screen dirty or thin; 



(d) If an approximately square room is used, then the 

 mirror and other metallic screens will appear very dark and un- 

 satisfactory for the spectators outside of an angle of greater than 

 15 to 20 degrees from the axis, and the farther outside the 15 de- 

 gree position the darker will appear the screen image (fig. 247). 



(e) The objective and second element of the condenser may be 

 improperly proportioned, i. e., focal lengths too different ( 89-90). 



2. Oppressive in the room. Too little fresh air. 



3. Room lights shining in the eyes of the spectators. Not 

 properly placed or shaded. 



4. Distorted image. The screen and the axial ray from the 

 projection apparatus not at right angles. 



5 . The details of the picture not visible for the spectators on the 

 back seats. The objective is of too long a focus and it does not 

 magnify enough. Use a shorter focus objective. 



6. The screen picture altogether too large. Too short a focus 

 objective; use one of longer focus, and adapt the condenser to it. 



7. There is a glare in the room from the ceiling or walls or both. 

 The paint used in finishing is shiny, not dull and flat. Use more 

 turpentine and less oil in the paint. 



8. The room too dark. Use more room lights properly placed 

 and shaded. 



