The best time for observation is in mid-morning and in mid- 

 afternoon when the errors of refraction are comparatively small and 

 when the sun is changing rapidly enough in both altitude and 

 azimuth to insure the best results. 



It is necessary to know the latitude of the place to the nearest 

 minute. This is found by measuring the altitude of the sun at 

 apparent noon, subtracting the refraction and adding parallax cor- 

 rection. From this, the zenith distance can be computed by sub- 

 tracting the result from 90. The latitude can then be determined 

 by the algebraic sum of the zenith distance and the declination. 

 (Refer to p. 168.) 



Fig. 88 



The appliances that have been contrived to facilitate direct 

 observation on the sun include the eyepiece cap with combination 



shutter and ray filter as shown 

 in Fig. 89, the eyepiece prism 

 for the steeper sights, with the 

 swiveled moderation glass as 

 shown above, and discussed 

 on p. 191 and the Davis Solar 

 Screen, (Fig. 88) designed by 

 J. B. Davis, erstwhile Prof. 

 Civ. Eng. at the Univ. of 

 Mich. The eye cap with ray 

 filter is furnished with our 

 Tachymeters and Theodolites 

 without extra charge when 

 specified; but the Davis Screen 



Fig. 89 



is made only to order because many engineers prefer to use a white 

 card, or the page of a notebook, to catch the projected image of the 

 sun and cross wires. 



152 



