228 



NA TURE 



{July 23. 1874 



Aristotle, in the thought that comets were not celestial 

 bodies, but mere sublunary meteors ; and now it was 

 discovered, by substituting observation for the word of 

 the master, that they journeyed far above the orbits of 

 Mercury and Venus, without being in the least incom- 

 moded by the crystalline spheres of the (irmament in 

 which the old astronomy incrusted its planets and stars. 

 From the time of Newton comets were at last embraced, 



so far as the movement of the nucleus was concerned, 

 in the theory of attraction, and consequently in pla- 

 netary astronomy, with this single difference, that they 

 described around the sun ellipses enormously elongated, 

 almost parabolic, instead of ellipses almost circular, 

 like the planets. Then astronomers observed carefully 

 the successive positions of these nuclei, and calculated 

 their orbits, but without attending to the figure of the 



comets themselves, although the invention of the tele- 

 scope must have already revealed a number of curious 

 phenomena which escaped the naked eye. During this 

 period astronomers restricted themselves to representing 

 the comet by a small circle, the centre of which alone 

 was of importance, for there was the centre of gravity to 

 which the laws of Kepler applied, and tlie calculation of 

 the elements of the orbit. As to the tail, which attracted 



Fig. 3. 



no attention, they figured it very simply by some feathery 

 traces attached to the nucleus. In all this there is 

 nothing to attract attention now, any more than the 

 dragons of the astrologers. It was no longer now a 

 superstitious prejudice which took from astronomers the 

 desire to closely examine the facts ; it was a preconceived 

 idea, an elev.itcd idea, no d.^ubt, but too absolute, accord- 

 ing to which the only force to be regarded in the celestial 



spaces was attraction. At bottom it was vaguely felt that 

 the figures of comets were irreconcilable with this ruling 

 hypothesis ; and this was sufficient, for the eye was 

 brought to bear by preference upon the subject the 

 most attainable by the reigning theories. 



Leaving aside the rude drawings of the six-tailed 

 comet of 1744, by Chdzeaux, and those which Messier 

 made by rule and compass, we must come down to the 

 two Herschels before we find trustworthy observations on 

 the form of comets ; the beautiful drawings of the comets 

 of 181 1 and 1835 are even now of use to science. Astro- 

 nomers had at last learned, from the example of Gibers 

 and Bessel, the high importance of these phenomena, 

 which reveal to us more than a new world, since they tell 

 us of a new force in the universe. At present the figure 

 of comets has become the subject of the most earnest 

 research, and the drawings of the beautiful comet of 

 Donati (1858) which 1 am about to show you will give 

 you an idea of the change which, in this respect, has 

 taken place in the minds of astronomers. I can confidently 

 vouch for their fidelity, for, while Bond was executing 

 these drawings at the Cambridge (U.S.) Observatory, by 

 means of a telescope of great power, 1 followed the same 

 body at Paris with the first telescope which Foucault 

 constructed on his new system, and it appears to me 

 while looking with you on these drawings of Bond, as 

 if I still had that wonderful comet before my eyes. 



I sh.all endeavour first to give an exact idea of the suc- 

 cessive metamorphoses which comets present during the 

 course of their appearance, taking as a type a comet 

 which has been perfectly studied — that of Donati. Let 

 us remember that these bodies describe around the sun 

 ellipses extremely elongated, of which the sun occupies 

 the focus ; that the point nearest the sun is called the 

 perihelion, while the most distant point (in a truly para- 

 bolic orbit this woald be infinite) is called the iip/ieiion. 

 Unlike the planets, which descrilse orbits almost circular, 

 and remain always at nearly the same distance from the 

 sun, comets, in general, come to us from regions much 

 more distant than the most remote planets ; but they 

 only become visible, even to the telescope, in the part of 

 their orbit which is nearest to the sun. After their pas- 

 sage at parihelion, their distance from the sun becomes 

 greater and greater, and soon they cease to be visible. I 

 do not believe that any comet has been seen beyond the 

 orbit of Jupiter. It is assuredly not on account of their 

 smallness that they thus escape our notice in regions 

 where the most distant planets, Saturn, Uranus, and 

 Neptune, shine so clearly with the light which they 

 borrow from the sun ; this is because the rare and nebu- 

 lous matter of comets reflect much less light tiian the solid 

 and compact surface of the planets of which we speak, 

 much less even than the smallest cloud of our atmo- 

 sphere. 



When they are seen far from the sun through a tele- 

 scope, they appear like rounded nebulosities, but vaguely 

 defined, presenting at the centre a condensation suffi- 

 ciently marked, which is called the itueleiis : it is this 

 nucleus, more brilliant than any other part, whose posi- 

 tion astronomers observe. Fig. 3, representing Donati's 

 comet at the time of its discovery, June 5, 1S5S, gives a 

 sufficient idea of the aspects of all comets when they are 

 at a great distance from the sun. 



At a later period, when the comet is approaching its 

 perihelion, it sensibly lengthens out in the direction of 

 the radius vector, i.e. in the direction of an imaginary line 

 which would join the comet and the sun ; but then the 

 bright nucleus is no longer found in the centre of the 

 figure, but is situated excentrically on the side nearest to 

 the sun, as is shown in Fig. 4. 



Later still, the tail is formed, and is developed more and 

 more, like an opened fan, while the nucleus shines with 

 a more vivid brightness. The comet becomes visible 

 to the naked eye as in Fig. 5. J 



