CHAP. I.] 



HELIOSTAT 



39 



the disc rod as nearly as may be, for elevation or depression. 

 Then, slightly loosening the screw clamping the arm, finally 

 adjust the latter, so that the object, as regarded through the 

 hole in the mirror, is obscured by the white cardboard disc in 

 the centre of the ring. By turning the mirror so that the dark 

 shade caused by the blind spot is thro^vn on to the disc, the 

 flash will be truly directed, and must be kept so by slight 

 alterations of the position of the mirror, which should there- 

 fore be clamped only sufficiently to hold it steady, and yet 



Fig- 7- 



LOOKING-GLASS AS FITTED BY BLACKSMITH FOR HELIOSTAT. 



a. Sliding collar carrying arm m, revolving round s ; h, wooden ring, painted 

 black, with cross-wires and white cardboard centre, sliding vertically by means 

 of rod through arm m ; c, iron frame to hold mirror, fitting into socket in top 

 of standard s ; s, iron standard with fixed tripod legs ; d, blind spot in 

 mirror ; e, screw for clamping mirror frame ; /, screw for clamping arm ; 

 ;/, screw for clamping ring rod. 



admit of gentle movement. The shadow of the blind spot 

 should be slightly smaller than the disc, so as to ensure having 

 it truly in the centre of the latter. 



The mirror must be of the best glass, with its faces parallel, uecess^^ 

 or the shadow of the bUnd spot will be very indistinct when 

 the mirror is at a large angle, and also the beam of light will 

 be dispersed before it has traversed many miles. 



It is well to have the mirror a fair size, say 6 inches square, Size of 

 as in practice it will be found generally necessary, in order to ""^' 



