DISTANCE AND DIMENSIONS OF THE SUN. 411 



in the camera; and a small plate of glass ruled with squares was 

 placed at the focus of the telescope and photographed with the sun's 

 image, furnishing a set of reference-lines, which give the means of de- 

 tecting and allowing for any distortion caused by the enlarging lenses. 



The Americans and French, on the other hand, preferred to make 

 the picture of full size, without the intervention of any enlarging 

 lens : as this requires an object-glass with a focal length of thirty or 

 forty feet, which could not be easily pointed at the sun, a plan proposed 

 first, I believe, by M. Laussedat, but also independently by our own 

 Prof. Winlock, was adopted. The telescope is placed horizontal, and 

 the rays are reflected into the object-glass by a plane mirror suitably 

 mounted. The French used mirrors of silvered glass, and took their 

 pictures (about two and a half inches in diameter) by the old daguerre- 

 otype process on silvered plates of copper, in order to avoid the risk 

 of collodion-contraction. With the silvered mirror the time of ex- 

 posure is so short that no clock-work is required. The Americans 

 used unsilvered mirrors, in order to avoid any distorting action of the 

 sun's rays upon the form of the mirror. This, of course, made the 

 light feebler, and the time of exposure longer, so that a clock-work 

 movement of the mirror was needed to keep the image from changing 

 its place on the plate during the exposure, which, however, never ex- 

 ceeded half a second. The American pictures were taken by the or- 

 dinary wet process on glass, and were about four inches in diameter. 

 Just in front of the sensitive plate, at a distance of about one-eighth 

 of an inch, was placed a reticle, or a plate of glass ruled in squares, 

 and between this and the collodion-plate hung a fine silver wire sus- 

 pending a plumb-bob. Thus the finished negative was marked into 

 squares, and also bore the image of the plumb-line, which, of course, 

 indicated precisely the direction of the vertical. The Americans also 

 placed the photographic telescope exactly in line with a meridian 

 instrument, and so determined, with the extremest precision, the 

 direction in which it was pointed. Knowing this, and the time at 

 which any picture was taken, it becomes possible, with the help of the 

 plumb-line image, to determine precisely the orientation of the picture 

 an advantage possessed by the American pictures alone, and making 

 their value nearly twice as great as otherwise it would have been. 



The following figure is a representation of one of the American pho- 

 tographs reduced about one-half. V is the image of Venus, which on 

 the actual plate is about one-seventh of an inch in diameter ; a a' is the 

 image of the plumb-line. The centre of the reticle is marked by the 

 little cross, and the word " China," written on the reticle-plate with a 

 diamond and, of course, copied on the photograph indicates that it 

 is one of the Peking pictures. Its number in the series is given in the 

 right-hand upper corner. About 90 such pictures were obtained at 

 Peking during the transit, and about 350 at all the eight American 

 stations, the work being much interfered with by unfavorable weather 



