CH. XII] 



POSITION OF PROJECTION APPARATUS 



449 



614. Level room with the apparatus near the screen. It is 



sometimes desirable to put the apparatus near the screen. Then 

 provision must be made by removing some of the seats if the center 

 aisle is not wide enough. 



The apparatus must usually be raised somewhat also, and some- 

 times the objective inclined more or less upward. In case it is 

 desired to have the apparatus very near the screen it must be 

 pointed upward considerably and then the screen should be hinged 

 at the bottom so that it can be inclined toward the lantern till it is 

 perpendicular to the optic axis. The simplest way to fix the 

 screen in any position, and to change the position is by means of 

 ropes and pulleys at the top. 



FIG. 241. 



LECTURE ROOM WITH RISING SEATS, AND THE LANTERN IN THE 

 MIDDLE OF THE ROOM, NOT AT THE BACK. 



B Black-board lighted by the hidden lights (H L) behind a curved metal 

 shield. 



L T Lecturer's table in front of the audience. 



Ml E The magic lantern (Ml) ; its rays shown in full lines, and the episcope 

 or opaque lantern (E) with its rays shown extending from the mirror (M) in 

 dotted lines. 



Sc The screen for receiving the image. As the magic lantern must be 

 elevated the screen is tipped toward it to meet the axial ray at right angles. 



For such a position of the magic lantern the projection objective must be of 

 shorter focus to give the desired size of image than when the lantern is at the 

 back of the room ( 636). 



