PHOTOGRAPHY IN THE SERVICE OF ASTRONOMY. 48*J 



year the negative of the Pleiades, which is well worth the troal)k^ The 

 iinj)ressioiKs of 1888, obtained with very sensitive i)hite8 and an expo- 

 snre of four hours, have revealed with surprising clearness the diffuse 

 mass of cosmical matter which envelopes this constellation, and of 

 which the nebulae of Maia and of Merope are oidy the most luminous 

 parts. A curious and very unexpected peculiarity is a rectilinear fila- 

 ment of nebulous matter which jjroceeds from the principal mass, over 

 a length of 40' of arc and a breadth of 3" to 4" only ; it encoun- 

 ters on its course seven stars which it unites together as beads of 

 a chaplet. A second line, similar but shorter, exists in the midst of 

 the nebulous mass. This new negative contains besides twice as many 

 stars as the tirst, about 2,000. The chart of the Pleiades of Mr. C 

 Wolf, which consumed several years of labor, contains only 671. 



Mr. Pickering has entered into the same path, and very recently his 

 l)]atcs have revealed the existence of five or six new nebuloi in difterent 

 regions of the sky. Finally, some months since, Mr. Eoberts commu- 

 nicated to the Astronomical Society in London photographs of the ellip- 

 tical nebula of Andromeda, which are indeed a revelation. That whicli 

 seemed an unformed mass of cosmical matter, traversed by irregular 

 fissures, appeared now as a solar system in embryo; rings are dis- 

 tinguished in it, which are detached from the central mass, as is re- 

 quired by the hypothesis of La^dace, and two satellites in course of 

 formation, whose relative positions must have undergone some changes 

 since the epoch of the observations of Bond. Photography renders 

 thus intelligible a structure which sketches are inclined to conceal. 



The success obtained in this field can depend, in a certain measure, 

 upon a particular photogenic power of nebula*; but it is explained 

 especially by this fact, that the sensitive plate is not dazzled by the 

 vicinage of more brilliant objects. The nebula which surrounds the 

 variable star Eta Argus, was invisible when- this star appeared to be 

 the first magnitude, and was discovered oidy when the star which 

 eclipses it caused it to descend to the fourth order (it is now the seventh 

 magnitude). 



There is found to be an advantage of the same order in the applica- 

 tion of photography in the registering of j)henomena instantaneous or 

 of very short duration, like eclipses, occultations, meridian transits, 

 where the cool-headed sensitive plate shields us from the trouble, and 

 from errors inseparable from a precii)ilate observation. A great num- 

 ber of total solar eclipses, also the two transits of Venus, 1874 and ]SSi>, 

 have already been observed by tliis means. The measures ol' numerous 

 negatives taken by the French expeditions have been confided to a per- 

 sonnel of the gentler sex, under the direction of IVIr. Boucpiet de La 

 Grye; they are completed, and the calculations are in a very advanced 

 state. 



We will limit here this rapid review of the services which photog- 

 raph}' has rendered to astronomy, or which it is to render to it alter a 



