Intelligence and Miscellaneous Articles, 



459 



that the determinations of two points take place not only mutually 

 but simultaneously. To make this possible, I have devised a helio- 

 trope which is placed upon the tube of a theodolite telescope, and 

 reflects the sunlight along the axis of the telescope. By means of 

 it the axes of the two telescopes are brought into the same straight 

 line when each observer sees the light of the sun reflected from the 

 other's instrument. 



The instrument (see adjacent figure) consists of two mirrors, a and 



x i 



---?- 



— -?- 



b, which form angles of 45° with the axes x and rj. The mirror a can 

 rotate about the axis of 77, b with the entire piece about the axis of x. 

 By these two rotations it can be arranged that rays which fall from 

 any point of the sphere on a are reflected at b at an angle of 45°, and 

 in the direction towards x. But in order to see in the telescope 

 whether the sun's rays are reflected towards x, an image of the sun 

 is needed which shall be 180° from the point that is to receive light 

 from the heliotrope. This is effected when the mirror b consists of two 

 halves, a and (3, which are in the same plane, but fit against each 

 other with vertically ground edges. This line yy' must make with 

 the surface of the mirror exactly an angle of 90° : it must not be 

 polished, but must only be rubbed smooth, otherwise it produces too 

 intense an image of the sun in the telescope ; for the rays which 

 proceed from 77 to 77' are reflected from the covered mirror b towards x. 

 In the middle of the mirror, however, the coating is taken off, so 

 that a small portion of the rays from 77 which goes through the plate 

 glass towards 77' meets the edge yy of the mirror b, and being re- 

 flected towards x , the image of the sun is formed in the focus of the 

 object-glass. — Poggendorffs Annalen, September 1865. 



2H2 



