Barker.] U5b [Dec. 1, 



second and much superior photograph of this nebula was secured after an 

 exposure of 104 minutes. And finally, a year later, on the 14th of March, 

 1882, he succeeded in making a successful exposure of 137 minutes, and 

 in producing a most superb photograph, which showed stars of the 13.7 

 magnitude, invisible to the eye, and in which the faint outlying regions of 

 the nebula itself were clearly and beautifully shown. This unrivaled 

 photograph, by far the most brilliant success yet achieved by celestial 

 photography, will ever have a very high astronomical value, since by 

 a comparison with it of photographs of this nebula, taken many 

 years subsequently, changes which are going on in it may be traced 

 and their history written. Ordinarily the photograph of a spectrum 

 is more diflBcult than one of the object itself. But in this case it is 

 not so. The spectrum being of bright lines, the light is localized and 

 readily impresses the plate. Moreover, any error in the rate of the clock 

 or any tremors of the instrument, which are fatal to the nebula, count for 

 little in photographing its spectrum ; since the image is thereby simply 

 shifted off the slit and no injury results to the definition. Many excellent 

 photographs of the spectrum of the nebula in Orion were obtained by Dr. 

 Draper, however, the chief interest in which consists in the fact that be 

 side the characteristic bright lines, there are traces of continuous spectrum 

 in various parts of the nebula, suggesting the beginning of condensation. 

 Beside the work done at his observatory at Hastings, which may be 

 called astronomical work proper. Dr. Draper occupied himself with col- 

 lateral questions of not less importance, in the admirably equipi)ed physi- 

 cal laboratory he had built in connection with his residence in New York 

 City. It was here, in 1873, that he made the exquisite, and to this dayun- 

 equaled photograph of the diffraction spectrum. The region from wave- 

 length 4350, below G, to wave-length 3440 near O, was contained upon a 

 single plate. The Roman astronomer Secchi reproduced this photograph 

 as a steel plate for his great work on the Sun, and the British Association, 

 in 1880, endorsed it as the best known standard spectrum by publishing a 

 lithograph of it in their Proceedings. The grating used to produce this 

 photograph was one of Mr. Rutherfurd's superb plates, ruled with 6481 

 lines to the inch. It was in his New York laboratory, too, that he made 

 the most important discovery of his life, perhaps ; that of the existence ot 

 oxygen in the sun. After months of laborious and costly experiment, he 

 succeeded, in 1876, in photographing the solar spectrum and the spectrum 

 of an incandescent gas upon the same plate, with their edges in complete 

 contact ; thus enabling the coincidence or non-coincidence of the lines in the 

 two spectra to be established beyond a doubt. On examining the spectrum 

 of oxygen thus photographed, he saw that while the lines of the iron and 

 the aluminum used as electrodes, coincided, as they sliould do, witli their 

 proper dark lines in the sun's spectrum, the lines of oxygen agreed with 

 bright solar lines. Whence the important conclusion announced by him, 

 1st, that oxygen actuallj^ existed in the sun, now for the first time proved ; 

 and, 2d, that this gas exists tliere under conditions either of tempera- 



