140 HELIOSTATS FOR THE MAGIC LANTERN [Cn, VI 



from east to west. In order that the sun's rays may shine in one 

 place continuously it is necessary to counterbalance in some way 

 the apparent motion of the sun. 



If one holds a plane mirror in the hands, it is possible to keep a 

 spot of sunlight on one place indefinitely by making slight changes 

 in the position of the mirror to correspond with the changes in 

 apparent position of the sun. This is possible from the law of 

 reflection: "The angle of incidence and the angle of reflection are 

 equal." (See fig. 80 and Chap. XIV, 794). 



A heliostat is then simply a mechanism for holding the mirror 

 so that the sun's rays may be reflected in a constant direction. As 

 this seems to make the sun stand still, the name is appropriate. It 

 was given by the original inventor, s'Gravesande, 1742 (fig. 77). 



There are three principal forms of heliostats : 



(1) The hand-regulated heliostat or porte-lumiere with one 

 mirror and a double movement up and down, and on the axis, so 

 that it may be made to follow accurately the sun's apparent motion 



(fig. 75). ' 



(2) A heliostat with one mirror, in which the movements of the 

 mirror are brought about by clock-work (fig. 77-79). 



(3) A Heliostat with two mirrors. One mirror is attached to 

 the end or side of the clock-shaft. The other mirror is not con- 

 nected with the clock-work. The second mirror serves to reflect 

 the beam from the movable mirror in the desired direction, and 

 is set by hand, once for all, at the beginning of the experiment. 



The clock-shaft rotates once in 24 hours with the single mirror 

 heliostats (fig. 77-79), and also with the two mirror heliostat 

 with the mirror at the end of the clock-shaft (fig. Si). 



When the mirror is attached parallel to the clock-shaft (fig. 82), 

 the clock-shaft rotates once in 48 hours (fig. 82, A, B, C). 



INSTALLATION AND USE OF A HAND-REGULATED HELIOSTAT OR 



PORTE-LUMIKRE 



234. The hand-regulated heliostat or portc-lumiere consists 

 of a plane mirror so mounted that it can move on two axes. The 

 mirror should be about 15 x 30 cm. (6 x 12 in.) in size and sup- 



