CHAP, i.] MIRRORS AND REFLECTING INSTRUMENTS. 215 



round the point 0, or that wished to be given to the reflected ray. 

 This rod is hollow and carries a fork to which is fixed the mirror, 

 m'm'j which can also turn round OE and BC. Another rod CD, 

 arranged in the plane of the mirror, is joined to a ring at D, at the 

 extremity of a rod OD equal to OC. A ray S'C, which falls on 

 the mirror parallel to SO will be reflected in the direction CR This 

 is the arrangement of Gambey's heliostat. 



4th. The mirror is supported at C by an upright, on which it can 



FIG. 152. Foucault's heliostat 



be moved in every direction. It is regulated by another rod CB, 

 normal to its surface, and connected at B to a ring fixed on OB at a 

 distance OB = OC. In its plane a third rod CD has a slot, in which 

 the prolongation OD of OB can slide. The two triangles, OCD and 

 OBC, are always isosceles, so that the normal to the mirror CB is 

 parallel with ON, the bisectrix of the angle of the incident and 

 reflected rays. This is the principle of Foucault's heliostat. 



