15.2 HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI 



INSTALLATION AND USE OF TWO-MIRROR HELIOSTATS 



243. Heliostat with the mirror at the end of the clock-shaft. 



Place the heliostat in a position either inside a room or outside a 

 window where the full light of the sun can fall upon the movable 

 mirror. The stand supporting the clock-work, etc., must be made 

 level, and set in a north and south direction (fig. 81). 



Elevate the clock-shaft above the level to an angle equal to the 

 latitude of the place where it is to be used. One can use a good 

 protractor for this. The clock-shaft will then point toward the 

 north star, and be parallel with the earth's axis (fig. 85). 



This form of heliostat often has the clock-shaft in a fixed position 

 for cheapness of construction (fig. 81). If such a heliostat is 

 purchased, the manufacturer must know the latitude of the place 

 where it is to be used, then he will give the proper inclination to the 

 clock-shaft so that when the instrument is arranged in a north and 

 south line the shaft will point toward the north star. 



244. Arranging the movable mirror. The mirror is fixed to 

 the end of the shaft by a collar which permits it to rotate around 

 the shaft. It is also held in a kind of fork, which permits the 

 mirror to be raised and lowered in a way similar to the hinge 

 movement of the porte-lumiere (fig. 75). 



For setting this mirror so that the clock-work will cause it to 

 throw a beam of light in one direction continuously, it is necessary 

 first of all to set the mirror for the local time. This is done by the 

 use of a perforated screen admitting a narrow pencil of light from 

 the sun. This screen is so placed that the pencil of light falls upon 

 the mirror. The mirror is then turned by loosening the clamp 

 (fig. 8 1 c) and rotating it on the shaft, and by tipping it in the fork 

 until the pencil of light is reflected back along its path through the 

 hole again. 



Then the clamp is tightened and the screen removed. The 

 mirror is now tipped in the fork until the light is reflected from it 

 directly in line with the clock-shaft, i. c., directly toward the north 

 star (fig. 81 N. P.). The easiest way to do this is to take a piece 

 of white cardboard with parallel black lines on it and place it 



