144 



HELIOSTATS FOR THE MAGIC LANTERN 



[Cn. VI 



hinges more and more until at noon it will be nearly horizontal 

 (fig. 76). In the afternoon, as the sun moves toward the west, the 

 mirror must be rotated to follow it. At the same time it must be 

 turned more and more on its hinges until late in the afternoon, it 

 will be at the same angle as in the morning, and rotated as far 

 toward the west as it was toward the east in the earlier part of the 

 day (fig. 76). 



(B) For an eastern exposure In this position, the axis of the 

 entire instrument is preferably east and west with the objective 

 pointing westward. The earlier the time the more nearly hori- 



North 



FIG. 76. DIAGRAM SHOWING THE POSITION OF THE MIRROR NECESSARY 

 TO RELFECT THE SUNLIGHT DIRECTLY NORTH AT THREE DIFFERENT 

 PERIODS OF THE DAY (6A.M.; 12 M.; 6 p. M.). 



The diagram is for the latitude of Ithaca and at the season of the equinox 

 when the sun seems to rise directly in the east and set directly in the west. In 

 the morning thu mirror is turned toward the east at an angle of 45 and inclined 

 about 10 toward the south. In the evening it is turned similarly toward the 

 west and south. 



At noon the mirror is raised on its hinges about 28 above the horizontal. At 

 all intermediate points the mirror must be set accordingly: that is, so that it 

 will reflect the sun directly north. 



The diagram also shows the apparent course of the sun from sunrise to 

 sunset. 



