PHOTOGRAPHY IN TIIK SERVICE OF ASTRONOMY. 485- 



arics of the solar cortei;e interest astronomers in more tliaii one (M)ii!iec- 

 tioii; indeed tbey are watched after almost for tlu' i)iii[>(),si' ol avoulin*;" 

 losiu*;;- them after leaving discovered and inscribed them upon the reg- 

 ister of the planetary system. This happens however from time to 

 time, when the first observations have not been sufficiently luimerous 

 1o lix very securely the elements of the orbit; there are at present a 

 score of these bodies which are wanting at roll-call. 



The great diversity in the form and situation of their orbits opens 

 to young astronomers a field for mathematical exercise, and raises at 

 times arduous problems. In order to arrive at a determination of their 

 feeble masses, which we have been able to estimate only from their 

 brightness, it is necessary to be able to establish, for example, the mu- 

 tual perturbations of two asteroids passing very near one another, so 

 that their reciprocal attraction becomes sensible aside from that of the 

 sun. It is a matter of interest then to predict the coming near to each 

 other or physical conjunctions of the asteroids; but the proximities 

 worthy of being noted are rather rare, or at least they occur only be- 

 tween the orbits and not between the planets.* Perhaps some day the 

 passing of a comet through the belt of asteroids will offer us other means 

 of estimating the power of the attraction of these p gmies. In return, 

 the perturbations which they experience themselves on the part of Ju- 

 l)iter are sometimes \:ery sensible, and they have already served (nota- 

 bly those of Themis and Amphitrite) in verifying the value of the mass 

 of this planet, which represents a little less than a thousandth of the 

 mass of the Sun. The one of the three planets discovered in the month 

 of last October, by Mr. Palisa (it has received the number 279, and the 

 name Thule),is particularly interesting in this regard, for its mean dis- 

 hinvA^ (4.3) surpasses that of all the known asteroids, and permits it to 

 ai)proach near enough to Jupiter to be very strongly disturbed in its 

 course. These are some of the reasons which make us think that i)ho- 

 togra])hy, in facilitating much the search for small planets, will not 

 serve solely for swelling the statistics of the solar system, 



A discovery infinitely more interesting would be, however, that of the 

 trans-i^eptunian planet, which has not ceased to haunt the imagination 

 of astronomers. For nothing proves that Neptune must be the last 

 term of the series of i)lanets which gravitate around the sun. We know 

 that Le Verrier, in 1840, had reached a determination of the position of 

 this star by the aid of the errors or residuals of the Tables of Uranus, 

 which amounted to 130", and which he attributed with reason to the 

 perturbations produced by an unknown planet. The day when he 

 was able to announce to the Acadenjy of Sciences that Mr. Galle had 

 come upon this planet in the indicated place, he added : " This success 



* Tlie shortest distance between the orbits of Thetis ami Belloiia is estimated at 

 ;in,000 kilometers; for Clytia and Nemesis, tliis distance is 11'), 000 idiometers, aud the 

 two i)lanets are found at d'A),000 kilometers apart in the month of August, Idd'J ; this 

 is twice and a half the distance of the moon from the earth. 



