This telescope was frequently taken on solar eclipse expeditions : as men- 

 tioned above, George Clark and John Whipple used it photographically 

 at Shelbyville in 1869. Winlock was anxious to show, from these 1869 

 photographs, that pictures taken in the principal focus of a telescope are 

 less distorted than those taken after enlargement by the eyepiece. To this 

 end he directed the Clarks to construct a micrometer to measure the 

 distance viewed under a microscope between the centers of the photo- 

 graphic images of the sun and moon. The results indicated the great 

 accuracy of this method of observation, 112 and most photographs of the 

 1 870 eclipse were taken in the principal focus. 



Shortly after the eclipse Winlock designed, and the Clarks constructed, 

 a fixed horizontal telescope for solar photography. This instrument was 

 so successful that a modified version of it was chosen for the official 

 American photographic observations of the Transits of Venus (q.v.) of 

 1874 and 1882. Sunlight was reflected to the objective by a flat mirror. 

 Rather than using a heliostat, whose irregular motion would cause some 

 distortion, Winlock manually adjusted the mirror before each exposure. 

 A fast moving shutter, to give as short an exposure to the glaring sun 

 as possible, was the most difficult part of this camera. The Clarks figured 

 two simple 4-inch objectives : one visual, of 40 feet focus ; and one photo- 

 graphic, of 32^/2 feet focus. Winlock found, however, that they gave 

 equally good images. Their extreme focal lengths insured large solar 

 images and, like the aerial telescopes of the 17th century, lessened the 

 effects of the non-achromatism of the lens. 113 



The meridian circle Winlock designed for the Harvard Observatory 

 was mounted in 1870. The metal work was done by Troughton and 

 Simms of London, while the optical parts were figured by the Clarks. 

 This included the objectives of the principal telescope — 8}4 inches aper- 

 ture, and 9 feet 4 inches focus — and of the two collimators — the same 

 focus, but slightly smaller diameter. Fourteen years later William A. 

 Rogers measured the errors of graduation of this meridian circle by means 

 of electromagnetic clamps. George Clark designed and supervised con- 

 struction of the equipment and he participated in the measurements. 114 



112 See Winlock's report on the solar eclipse of 1869 in U.S. Coast Survey Report 

 (1869), pp. 124-125. 



113 Annals, Harvard College Observatory, vol. 8 (1877), pp. 35-42. 



114 William A. Rogers, "On the Original Graduation of the Harvard College 

 Meridian Circle in situ,^ Sidereal Messenger, vol. 3 (1884), pp. 306-311. 



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