156 



HELIOSTATS FOR THE MAGIC LANTERN 



[CH. VI 



M 2 The fixed mirror. This is adjusted at the beginning of the experiment 

 to reflect the light in the desired place and usually needs no attention during 

 the experiment. 



N P Continuation of the clock-shaft pointing toward the north pole. 



L 42.5 The angle made by the clock-shaft, and the horizon at Ithaca, 

 New York, U. S. A. It indicates the latitude of that place, and the elevation 

 of the clock-shaft to make it point toward the celestial north pole. 



S B Sunbeam admitted through a hole in the shutter. It strikes the first 

 mirror and is reflected to the second mirror, and from it in any desired direction. 



\2 M 



B 



FIG. 82 A, B, C. DIAGRAMS SHOWING THE POSITION OF THE FIRST 

 MIRROR OF THE HELIOSTAT (Fig. 82) AT DIFFERENT TIMES OF THE DAY 

 TO REFLECT THE SUNLIGHT CONSTANTLY IN THE SAME DIRECTION. 



The eye is supposed to be looking along the axis of the clock-shaft. It is to 

 be noted that between 6 A. M. and 6 P. M. (12 hours) the mirror has turned 

 through an angle of 90, and at this rate it takes 48 hours for the mirror to 

 make a complete revolution of 360. 



The arrows indicate the direction of the light before and after reflection from 

 the mirror. 



A Position of the mirror at 6 A. M. 



B Position of the mirror at 12 M. 



C Position of the mirror at 6 P. M. 



At intermediate periods the mirror will be in correspondingly intermediate 

 positions to reflect the sun constantly in the same direction, that is, the mirror 

 must follow the sun. 



This is one of the easiest heliostats to manage, as one needs to 

 know only the latitude and the north and south direction. The 

 arrangement of the two mirrors can be easily made at any time 

 and in any place by trial. 



HELIOSTATS ix THE SOUTHERN HEMISPHERE 



249. U]) to the present, the discussion has been with reference 

 to heliostats in the northern hemisphere. For those to be used in 

 the southern hemisphere certain modifications are necessary as seen 

 from the following considerations : 



