80 president's address — SECTION A. 



it produced. In 1874 Dr. Huggins tried to photograph the spectra 

 of planetary nebulae, but wiiiiout success, the instrument at his 

 command not beina; large enough. 



The year 1876 was an important epoch in the application of 

 photography to the astronomer's needs, for in that year gelatine 

 dry plates, which had been first put on the market in 1871, attained 

 such perfection that Dr. Hu^igins, after an extensive series of 

 tests comparing them with the best collodion films, gave the 

 preference to the new-fashioned dry plates, and therefore ex- 

 posures could be continued for hours, and even days, instead of 

 a few minutes, the possible limit for collodion plates. Dr. Huggins 

 used the ncAv plates to record ihe spectrum of Vega on December 

 21st that year ; it contained seven strong lines, all of them strongly 

 shaded at the sides, and two of them coinciding with the well- 

 known lines of hydrogen. Thus another advance was made; the 

 greatest number of lines previously photographed was four. 



Dr. H. Draper^" in 1877 announced his discovery of oxygen 

 in the sun in a paper read before the American Philosophical 

 Society. On July 20th he found, by photographing the spectrum, 

 a number of bright lines in the solar spectrum coinciding with 

 lines of oxj^gen, and said "We can no longer regard the solar 

 spectrum as a continuous spectrum with certain rays absorbed by 

 a layer of igniied metallic vapors, but as having also bright lines 

 and bands superposed on the background of the spectrum." 



In 1877 came another important advance. M. Janssen suc- 

 ceeded*' in photographing the sun, with extraordinary results. 

 The images were 12in. in diameter, and displayed remarkably 

 sharp details of the sun spots, willow leaves, lice grains, and 

 faculse. But the most remarkable result obtained — and which 

 was exclusively due to the improved photographic method — the 

 whole photosphere was covered with a fine granulation of very 

 varied forms, dimensions, and arrangements ; but the most re- 

 markable of all was the discovery of a fine photosplieric network 

 — " Reseau photosperique " The forms generally have rounded 

 contours, but some are rectilinear and others polygonal ; and in the 

 intervals of this network the rice grains are distributed and definitely 

 bounded, and in their interior (i.e , the net spaces) the " granules 

 are half obliterated, drawn out, and confused." This great step 

 in advance was obtained chiefly by improving the old flashing 

 shutter and reducing the time of exposure to -airoTyth part of a second. 



In 1878*^ Dr. H. Draper succeeded in getting very perfect 

 photographs of the solar eclipse in July of that year, showing that 

 the spectrum of the corona was similar to that of the sun — in other 

 words, the corona must be sunlight reflected from matter in the 

 neighborhood of the sun, and, if that accounts for the whole of its 

 light, then it would not be possible to photograph it apart from the 

 sun. The photograj^h was confirmed by the visual observations of 

 Professors IBarker and Morton, two of Dr. Draper's party. 



