PHOTOGRAPHY IN THE SERVICE OF ASTRONOMY, 487 



cated by Sir John Herschel iu reference to the pertiirbatioiKS of Uranus 

 due to Neptune. That whicli Uas fortified bini in this conviction is 

 the very probable ac(;i(lental agreement of his result with that to which 

 i\Ir. G, Forbes has been conducted by the consideration of a tendency 

 to grouping of the aphelia of periodic comets, whose distances from the 

 sun coincide more or less exactly with the mean distances of the larger 

 planets. Having found indeed seven comets whose aphelion distances 

 approach 100, and six whose aphelion distances approach 300 (the unit 

 being always the radius of the terrestrial orbit), Mr. Forbes concluded 

 from this that it is possible that there are two trans-]Srei)tuniau x)lauets 

 situated respectively at the distances 100 and 300, and whose powerful 

 attraction has acquired these comets for the solar system. The comets 

 thus captured would be able then to inform us of the actual position of 

 the planet to which we owe them, and which has in times past found 

 itself iu proximity to their aphelia. But having taken little from the 

 value of these premises, the numerical data upon which the calculations 

 of Mr. Forbes rest do not bear scrutiny. The agreement of the results of 

 Mr. Forbes and Mr. Todd signifies nothing when it is seen how these 

 results have been established. In spite of everything the trans-Nep- 

 tunian planet may indeed appear some day before our astonished gaze 

 upon one of the negatives which will serve to prepare the general chart 

 of the sky. 



Physical astronomy also sees new horizons to open out before it. I 

 will not speak here at length of the photographs of the sun and moon. 

 For a long time we have been able to see those which have great 

 beauty. We know with what success Mr. Janssen, at Meudon, pursues 

 the application of photography to the study of solar phenomena. Re- 

 searches of the same kind are made at Potsdam, and Mr. Wilsing, depend- 

 ing on a hundred plates for the positions of groups of facuhe, has arrived 

 at this unexpected conclusion, that (contrary to that which takes place 

 with the spots) the velocity of rotation of the facuhe is the same for all 

 the parallels, and equal to that of the equator. The retardation of the 

 motion of the spots explains indeed wliy those which originate at the 

 foot of a taenia i)roceed gradually in the direction of its paraUel as if 

 sown upon its course. It is i)robable that the unequal velocity which 

 the spots possess is limited by a rather thin layer of the solar envelope, 

 while the great mass turns solidly with the constant velocity of the 

 facuhe. 



We know the stereoscopic effects whicii are obtained with photo- 

 graphs of the moon, taken at two epochs suitably selected. Perfected 

 from time to time, these photographs will be of service in studying 

 more exactly the libration ; they will also cause the discovery of changes 

 which are occurring, perhaps, on the surface of our satellite, and which, 

 affirmed by some, contested by others, remain up to the present time 

 very doubtful. On the contrary, thereality of modifications, sometimes 

 rather sudden, appears to-day well verified foi- some planets. It suf- 



