CH. VI] 



HELIOSTATS FOR THE MAGIC LANTERN 



FIG. 82. 



TWO-MIRROR HELIOSTAT WITH THE MOVABLE MIRROR ATTACHED 

 PARALLEL TO THE CLOCK-SHAFT. 



This heliostat is adjustable for latitude and can be used anywhere in the 

 northern hemisphere, and by reversing the motion, in the southern hemis- 

 phere ( 253). 



C Clock-work mounted on a hinged plate. 



M 1 Rotating mirror attached to the side of the clock-shaft. From this 

 arrangement its plane would pass through the celestial north pole if extended. 



*To get this picture, the heliostat was set in the west window of Stimson Hall 

 at 2:30 P. M., May 20, 1912, and the mirrors arranged to receive and reflect a 

 small beam of sunlight as indicated. A black cord was extended from the 

 small hole in the shutter to the point on the first mirror receiving the sunbeam, 

 and from thence to the second mirror along the path of the sunbeam ; and from 

 the second mirror to a point on the screen receiving the sunbeam. The 

 apparatus was then photographed. To make the course of the sunbeam very 

 sharp for this cut its course was traced on the photograph by a right line pen. 

 The clock-shaft was also extended above and below and an arc of a circle added 

 between the clock-shaft and the horizon to indicate the angle of elevation of 

 the clock-shaft, corresponding with the latitude of Ithaca (42.5 North Lati- 

 tude). 



