PlIOTOGEAPHY IN THE .Sfc^KVlCE OF ASTKONOMY. 483 



to this class of researches. Mr. Pickering,* whose energy knows no 

 obstacles, has nndertaken a veritable spectroscopic revision of the sky. 

 In the tirst place a catalogue of the spectra of the stars visible to the 

 naked eye has been commenced. A second catalogue will contain 

 numerous spectra of faint stars, to the eighth magnitude. It is proposed 

 besides to make a detailed study of the spectra of the brightest stars, 

 of variable stars, and in general of all spectra which offer remark- 

 able peculiarities. A first list of 10,875 spectra is finished. This 

 autumn an expedition will be sent to the southern hemisphere, probably 

 Peru, to complete the work to the south pole. 



Mr. Pickering hopes also to draw to a successful termination a series 

 of photometric researches which have for an aim the comparison of 

 stellar magnitudes, furnished on the one side by, photograi)hy and on 

 the other by direct observation by the means of various photometers in 

 use. These researches reach a, tliousand stars near the pole, an equal 

 number taken in the neighborhood of the equator, and the stars visible 

 in the constellation of the Pleiades, the one of the best known in the 

 northern sky, and which offers the advantage of containing scarcely any 

 but white stars. It is also this constellation that Mr. J. Scheiuer has 

 selected for photometric experiments, the results of which he is about 

 to publish. 



These classes of researches will give the means of reducing the dif- 

 ferent scales to a common measure. We know already that the photo- 

 graphic scale is established by the diameters of the stellar disks. For 

 a given time of exposure the differences of the diameters will, in gen- 

 eral, be proportional to the differences of magnitude, such as result 

 from direct i)hotometric comparisons. With a little acquaintance the 

 estimation of the magnitudes could be without doubt reached during 

 the micrometric measures of the negatives, as astronomers estimate 

 them during the observation of transits. For a more precise determi- 

 nation all the stars on a negative could be referred to three or four 

 among them, of which the magnitudes could be measured by pho- 

 tometry. 



The processes in use permit in general the fixing of the magnitude t 

 of a star to about one-tenth, at least, for the first nine or ten magnitudes; 

 this is shown by the agreement of the published determinations by dif- 

 ferent observers. This is not so when exceptionally bright stars which 

 range above the first magnitude are concerned, or very faint stars below 

 the tenth, and the designations of the fifteenth, sixteenth, seventeenth 

 magnitudes do not have a precise meaning, only by virtue of definition, 

 by such and such an observer. We can, as is done at Paris, define them 

 by the length of exposure necessary to make the images appear, for this 

 time varies in the proportion of 1 to 10,000 from the sixth magnitude 



*E. C. Pickering, annual reports of the photograpljic studj' of stellar spectra. 

 tFrom ouo magnitude to auotUei the relative brightness dimiuishe.s (iu the wean) 

 jn the proportioo of I ; 0-4S. 



