484 PHOTOGKAPIIY IN THE SERVICE OF ASTRONOMY. 



to the extreme limit of visibility, aucl furnishes a scale the most ex- 

 tended. But it is necessary to" take account of the variable sensitive- 

 ness of the plates ; finally it is clear that the process founded upon the 

 estimation of the time of exposure is not favored in present applications 

 as much as the method which consists in the comparison of the disks 

 taken on the same negative. It is then the latter method which is 

 sought to be perfected, for it does not suffer from difiiculties when stars 

 gradually fainter are dealt with. The images, then, in forming have 

 dimensions already very appreciable, which increase only very little in 

 the first moments; the comparison of diameters can conduct to errone- 

 ous results if it is taken too soon. The progressive increase of the disks 

 has for a cause the irradiation which results from the interior illumina- 

 tion of the translucent gelatine at the point where the image is formed. 



III. 



The measurers of double stars are subject to a multitude of errors 

 which regard the difference of magnitude of the components, the incli- 

 nation of the line of the stars, etc., and which render the results ob- 

 tained by different observers comparable with difficulty. We meet 

 difiiculties of the same nature in micrometric measures of satellites, and 

 it is in all these cases that the intervention of photography promises to 

 increase greatly the accuracy and security of the results. The negatives 

 obtained by Messrs. Henry permit the pointings to be made with extra- 

 ordinary precision. The sensitive plate is not like the eye, dazzled by 

 the vicinage of a bright star; it remains attentive to the faintest 

 gleams. The satellite of Neptune, always visible with difficulty at 

 Paris, can be photographed in all parts of its orbit, even when it is 

 found at only 8 seconds from the planet. 



Satellites of new planets, hitherto unknown, will reveal their exist- 

 ence by the trace of their course in the midst of fixed stars. The ap- 

 parent displacement of a small i)lanet about the epoch of opposition, 

 that is to say at the moment when it a^jproaches nearest to the earth, 

 is in the mean one minute of arc in two hours, or 0'.5 per hour; upon 

 the negatives of the Paris observatory a trace is produced, in one hour 

 of exposure, of one-half of a millimeter. For the planet Pallas, which 

 is the eighth magnitude, this trace is found easily recognizable; but 

 Messrs. Henry think that it would still be appreciable for a planet of 

 the fourteenth or fifteenth magnitude, with a relative brightness four 

 or five hundred times fainter. 



The number of asteroids known has increased by several each year; 

 it reaches already 283. Thanks for the intervention of photograi)hy, 

 the search, hitherto very laborious, for these little bodies will become 

 so easy that we shall see them multiply too rapidly for the liking of the 

 calculators, and there will not be time enough to select their names. 

 In spite uf the iusignificaucc of their masses these humble supernumer- 



