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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 curtain with a hole in it, through which the instrument may be extended 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 having 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 condenser. 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