CH. VI] HELIOSTATS FOR THE MAGIC LANTERN 143 



236. Darkening the room. The room is darkened in the 

 usual way with curtains or shutters. The window where the 

 apparatus is to be placed must be darkened by a shutter or a cur- 

 tain with a hole in it, through which the instrument may be ex- 

 tended out into the sunshine and through which the sunshine 

 can be reflected into the room. 



The window frame must either be raised entire or one of the 

 panes must be hinged so that it can be opened when desired. One 

 can use the heliostat within the room utilizing the sunlight passing 

 through the window glass, but this is far less satisfactory than hav- 

 ing the heliostat out in the free air where the sun shines directly 

 upon it. 



Finally it must be possible to close the openings completely so 

 that the room may be made as dark as desired. 



237. Operation of the apparatus. In starting work at any 

 time the mirror is inclined on its hinges and rotated until the sun 

 shines upon it, and then until the light is reflected into the con- 

 denser. Finally some further slight changes may be necessary to 

 get the light accurately centered so that it will pass from the 

 condenser along the common axis to the objective and thence to 

 the screen (fig. 74). By changing the position of the mirror 

 slightly every three to five minutes to compensate for the apparent 

 motion of the sun, the light will continue to pass through the magic 

 lantern to the screen. 



238. Adjustments necessary for the different windows. 



(A) For a southern exposure For this exposure it is desirable 

 to have the entire outfit in a north and south direction with the 

 objective pointing toward the north. In the morning the mirror 

 is turned on its hinges to about 45 and then rotated toward the 

 east until it receives the light of the sun (fig. 76). It must then 

 be turned slightly by one or both of its possible movements until 

 the light is reflected in the desired direction. As the sun continues 

 to rise in the sky the mirror must be rotated on the axis from the 

 east toward the west to follow the apparent movement of the sun. 

 As the sun gets higher and higher the mirror must be turned on its 



