3i4 JOSEPH WINLOCK. 



immeciiately recognized the fact that the corona was broader in the 

 direction of the sun's equator than along the axis. He had arranged 

 for obtaining numerous views of the partial phases of the eclipse, iu 

 the hope of extracting from them valuable information as to the use of 

 photogi-aphy in observing the transits of Venus. To this end, he was 

 afterwards authorized by the Superintendent of the Coast Survey to 

 engage Messrs. Alvan Clark and Sons to construct a micTometer, 

 adapted to the nice measurement of distances and positions on the 

 photographic plates. The Annals of the Observatory will contain a 

 description and engraving of this micrometer, and an account of the 

 measures made with it, with various representations of the eclipse 

 copied from the photographs. 



At this time, no one except Mr. Winlock had succeeded in obtain- 

 ing a photograph of the corona during any solar eclipse. Although 

 his photographs were only ^ of an inch in diameter, they seemed to 

 promise measurements, made under a microscope, which would compare 

 favorably with the best that could be furnished by meridian instruments. 

 A larger image would be still better ; but this required a telescope 

 of formidable length, and diihcult to manipulate. To surmount this 

 obstacle, Mr. Winlock conceived the idea of a horizontal telescope, to 

 be fed by the liglit from a heliostat. lie was convinced that a trans- 

 parent reflector would be better than a silvered mirror, as it would 

 weaken the light, and supersede the necessity of making the time of 

 exposure inconveniently short. Moreover, as the instantaneous action 

 of the light was often sutficient, the heliostat was unnecessary. Soon 

 after his i-eturn from Kentucky he gave an order to the Messrs. Clark 

 and Sons for a lens of four inches in aperture and furty feet in focal 

 length, which, after some preliminary trials at their shop, was ready 

 for use at the Observatory in July, 1870 ; and, since then, has been in 

 constant employment for procuring photographs of the sun. The lens 

 is mounted upon one pier, the reflector u[)on another, and the camera 

 upon a third i)ier. The tube used for excluding the daylight is dis- 

 connected from the essential parts, so as not to disturb their stability. 



At the request of the Superintendent of the United States Coast Sur- 

 vey, Mr. Winlock organized and directed one of the parties sent to the 

 south of Europe to observe the total eclipse of the sun on December 

 22, 1870. He selected Jerez de la Frontera, near Cadiz, as a favor- 

 able station, and was assi>ted by one experienced officer of the survey, 

 by several eminent astronomers and physicists, and by one of his own 

 staff at the Observatory. Among the physical and astronomical instru- 

 ments which he prepared for this expedition was a lens of 32 J feet in 



