186 PROCEEDINGS OP THE AMERICAN ACADEMY 



XVI. 



THEORY OF THE HORIZONTAL PHOTOHELIOGRAPH, 



INCLUDING ITS APPLICATION TO THE DETERMINATION OF THE 

 SOLAR PARALLAX BY MEANS OF TRANSITS OF VENUS. 



Br Professor William Harkness, U. S. Navy. 



Presented March 15th, 1877. 



The terra "Horizontal Photolieliograph " is used to designate that 

 form of Photolieliograph which, it is believed, was first employed by 

 the late Professor Joseph Winlock; and which consists essentially of 

 a fixed telescope whose optical axis is accurately horizontal and in the 

 meridian, and whose objective is directed toward the same side of 

 the zenith as the elevated pole ; the sun's rays being reflected into the 

 telescope by a suitable heliostat. The sensitive plate for the reception 

 of the photogra|)hic image, is, of course, situated at the chemical focus 

 of the telescope ; the plane of the plate being perpendicular to, and its 

 centre coinciding with, the optical axis of tho telescope. 



S 



D, 



Fig. 1. 



The details of the construction of the horizontal photoheliograph, in 

 60 far as they are necessary for a proper understanding of the theory 

 of the instrument, are shown in Fig. 1. 



A is the heliostat mirror, consisting of a piece of highly polished, 

 but unsilvered glass, whose two surfaces make an angle of about 

 sixty minutes with each other. The front surface is worked as truly 

 plane as possible, and serves to reflect the solar rays through the 

 objective, to the photographic plate. In working the back surface no 

 particular pains are taken, and, on account of its inclination to the 



