CH. VI] HELIOSTATS FOR THE MAGIC LANTERN 151 



forms of single-mirror heliostats. In fig. 79, for example, the 

 position arm B-F can be raised or lowered and the entire arm can 

 be rotated around the column A. When the light is accurately 

 directed, all the clamps can be tightened and the clock-work should 

 cause the mirror to hold the light constantly in position. It will 

 be found much easier to center the light on one axis if the heliostat 

 is at about the same level as the condenser and objective. This 

 position can be secured by raising the heliostat or the lantern, 

 whichever is more convenient, provided the two are not on the 

 same level to start with. 



o 

 FIG. 80. PRINCIPLE OF THE UNIVERSAL HELIOSTAT SHOWN IN FIG. 79. 



O A The clock arm pointing directly towards the sun. 



B The position arm, pointing in the direction in which it is desired to 

 reflect the light. 



in The incident light parallel to A. 



rf The reflected light. 



A B The mirror. The mirror is perpendicular to the plane passing through 

 A , and B. 



N Perpendicular to the mirror A B. 



In order to prove that incident light parallel to A will be reflected from 

 the mirror parallel to B it is necessary to prove that A O, B and N are 

 in the same plane and that O N bisects the angle A B. The mirror being 

 perpendicular to the plane containing A , and B and the line O N perpendicu- 

 lar to A B must also be in this same plane. The triangle A O B is isosceles by 

 construction, as A O and B are made equal, hence the perpendicular to the 

 base must bisect the vertex angle. 



