the si/.e of the mirror is regulated to accord with the length of sight, 

 the image becomes too large and brilliant. The size of the image, 

 however, can be, and ought to be, regulated to suit the distance in 

 about the proportion of 1 inch per 20 miles. Our mirrors are l/i 

 inches across the shorter diameter. This would be ample for 25 

 miles. As the distance decreases, cardboard stops can be cut out 

 and pasted over the outer edge of the mirror. 



This precaution is taken to reduce the dazzling effect which 

 could otherwise be modified for city work with a sun glass shutter, as 

 illustrated in Fig. 89. The beamfrom the sun should be regarded as a 

 cone of light coming to a point at the center of the mirror. The sun 's 

 diameter is about 32', so that the projected beam will have the same 

 angular value. This is sufficient to subtend about 50 feet per mile, 

 or about 1000 feet in 20 miles. No very accurate adjustment for 

 centering on the theodolite station is necessary. 



The heliotroper has only to mount the apparatus over the center 

 of the monument. In the top of the tube supporting the mirror is 

 a small lens, and in the bottom of the tube is a white reflecting 

 surface. The heliotroper turns the tube toward the sun. When 

 the little lens has picked up the sun's rays and there is flashed back 

 from the white reflecting surface a miniature image of the sun, the 

 image should be centered in the small circular opening in the 

 center of the mirror and kept there. The lower portion of the 

 instrument being clamped, the mirror may now be tipped on its 

 pivots and turned equatorially with its standard until, looking 

 through the little orifice, the heliotroper has covered the instrument 

 station with the reflected beam, which can be distinctly seen pierc- 

 ing the atmosphere out beyond. This need not be done with great 

 exactness, for the divergent character of the reflected beam will 

 permit an error of 15' in setting. On this account the Steinheil 

 Heliotrope should rightfully be regarded as superior to all others. 



It is generally screwed into any convenient mounting in align- 

 ment back of the instrument to facilitate intercornunication from 

 the observation station, as suggested in Fig. 76; but if thought 

 desirable it could also be mounted interchangeably with the equator- 

 ial solar attachment and the auxiliary mining telescope, on the swivel 

 mount, as shown in Fig. 77 also in Figs. 98 and 106. By this means 

 signals can be exchanged either bytheMorsecodeor by a prearranged 

 system. When each station is provided with a heliotrope, they find 

 each other readily by slowly swinging the sunbeam in the genera! 

 known direction. Fig. 77 shows one of our latest designs in City 

 Triangulation and Tunm-l Theodolites with the heliotrope mounted 

 detachably. Fig. 76 also shows the supplementary mirror 

 which is screwed into some substantial mounting and used only 

 when the signal flash is to be projected onward in the same general 

 direction as the original sun beam. 



136 



