Jan. i, 1885] 



NA TURE 



J 95 



or treble, white or coloured, but only to examine the 

 nebulae of various degrees of condensation. Your wishes 

 being thus indicated, the astronomer will point out to you 

 the most characteristic objects, he will calculate their 

 exact positions, will prepare his most powerful telescope, 

 and then you will be able to make an interesting journey 

 into space. 



The nebula of Orion has not a clearly defined form ; 

 one region more brilliant than the rest can be distin- 

 guished, where the condensation of the chaotic matter is 

 rather far advanced. In all other parts the light is feeble, 

 and one can detect long streamers of matter of which it 

 is impossible to predict the action. 



The nebula of Andromeda is one of the most remark- 

 able objects in the heavens. It has an almost geometrical 

 form, and in the centre it exhibits a most distinct con- 

 densation. 



The nebula of Leo presents nebulous rings in course of 

 formation. 



Finally, the curious double nebula; of Virgo, Aquarius, 

 &c, are evidently very near their ultimate transformation 

 into stars. 



It would be easy to multiply the intermediate stages, 

 and to show, for example, some nebulous stars presenting 

 the penultimate phase of this series of transformations, 

 which commences with a feebly luminous mist without 

 shape, and finally arrives at one or many suns variously 

 connected. Needless to say, we are not present at these 

 transformations, but we are like the botanist who in the 

 forest studies the trees in their different degrees of deve- 

 lopment. Thus the creation of the universe is carried 

 on, so to speak, under our eyes. In the beginning nebula: 

 separated out from a universal chaos ; in the end, in- 

 candescent stars, or other globes so small that we cannot 

 see them, because their formation has produced so little 

 heat that their light is already extinct. 



Let us imagine that, owing to some cause of which 

 we shall presently speak, the spirals of a whirling nebula 

 are transformed into nebulous concentric rings, governed 

 by a common movement of rotation. In reality there 

 exist in the heavens objects of this description : for 

 example, the annular nebula in Lyra. 



If such as these are rare, it is because they usually do 

 not possess great stability. It is only a transitional form. 

 In reality, in virtue of the differences of linear speed 

 which predominate there, and because of the mutual 

 attraction of their parts, the least cause will lead to 

 . which, being obliged to follow somewhat the 

 same road with rather different speed, reunite and are 

 lost in a single nebulous mass, where, little by little, all 

 the material of the rings will be absorbed. This nebu- 

 lous mass, excited by a rotation in the same direction as 

 that of the ring, will in its turn give birth to a planet 

 surrounded by satellites revolving in the same direction 

 and in the same plane. 



We have a series of nebulous rings, some of which show 

 the eddying condensation which ends in a mass of planets. 

 At the same time the enormous quantity of material which 

 in the midst of the original nebula: was not used up in the 

 rings, has little by little reunited in the middle, very 

 slowly at first, but afterwards very quickly, giving rise to 

 a central globe, a Sun, turning on its own axis in the 

 same direction and in the same plane as the planets. 



We thus see how a slow whirling movement, more or 

 less indistinct, would be able to be governed so far as to 

 give rise to these circular rings, all of them concentric 

 and situated in the same plane. 



It is necessary and sufficient for this theory that the 

 solar nebula has been, in the first instance, spherical and 

 homogeneous. In such a mass of matter the internal 

 gravitj resulting from the attraction of all the molecules 

 varies in a direct ratio with the distance from the centre. 

 The particles or the small bodies which move in such a 

 medium, where the rarity is inconceivable, necessarily 



describe ellipses or circles round the centre in the 1 m 

 time, whatever may be their distance from that centre. 

 Thenceforth the existence of rings rotating in one piece, 

 with the same movement, is quite compatible with this 

 condition of gravity, and if a whirling motion has pre- 

 existed, some of these spirals, which are not so very 

 different from circles, will have little by little become 

 transformed into the rings previously described, owing to 

 the small amount of resistance at the centre. 



Let us take a step further. In virtue of the force of 

 attraction these rings tend generally to break up and to form 

 a nebulous spherical mass, which in the end contains all the 

 material of the ring. Now these secondary nebula: must 

 necessarily be endowed with the same direction of rotation 

 as that of the rings. Phenomena exactly like those of the 

 primary nebula will then take place ; that is to say, they 

 will resolve themselves into concentric rings, then into a 

 central globe. In their turn, the rings will be condensed 

 into other very small balls — satellites revolving round each 

 planet, always in the same direction, whilst the planet 

 will turn on its own axis exactly in the direction and in 

 the plane of these secondary rings. 



It is thus that these things have come about. By a 

 lucky chance some rings of the little secondary system of 

 Saturn have escaped destruction, and have not been 

 formed into satellites. I attribute their existence to the 

 extreme thinness of these rings and to their rapid 

 rotation. 



We should now have finished the explanation of the solar 

 system if this system did not offer a striking peculiarity, 

 apparently in complete contradiction with what has pre- 

 ceded. Of the eight large planets revolving round the sun 

 six have satellites, and thus form secondary worlds, exact 

 representations of the solar world which includes them. 

 After what I have said, all the rotations and revolutions 

 ought to be in the same direction, and, what is more, in 

 the "direct" direction. Now in the secondary worlds of the 

 two planets furthest off — those of Uranus and Xeptune — 

 the rotations and revolutions of the satellites are in the 

 opposite direction, that is to say, retrograde. 



Musi we believe that the theory that I have put before 

 you is false? It is not false, but it is incomplete. And 

 here we come to one of the most interesting points in the 

 history of science. Newton and Laplace believed that 

 all the rotations, all the revolutions must be in the same 

 direction. Laplace went further, and applied to this 

 question the theory of probabilities. In working on the 

 planets and satellites as known in his day, his analysis 

 showed that, if a new planet or satellite was discovered, 

 the chances were tens of thousands to one that the revo- 

 lution of this or that satellite, or the rotation of this or 

 that planet, would be direct, like all the others, and he 

 added that this probability is much greater than that of 

 historical events which we accept with the utmost con- 

 fidence. The study of the satellites of Uranus, and the 

 of the system of Neptune, however, has at once 

 destroyed this probability, and the celebrated cosmogony 

 of Laplace. This in fact by an ingenious process derives 

 all the planets from the sun, but it can only give to the 

 planets and satellites relations and revolutions in the 

 same direction from one end of the solar system to the 

 other, whilst in fact they are direct in the first half and 

 retrograde in the second. 



Let us actually complete our theory. In the primi- 

 tive nebula, homogeneous and spherical, where the 

 presence of rings revolving round the centre ought not 

 to alter anything in the law of interna! gravity, we have 

 seen that this gravity varies in a direct ratio with distance 

 from the centre. But, later, the sun was formed by the 

 reunion of all the matter not wanted for these rings ; this 

 has produced an empty space around it. Therefore the 

 law of gravitation in the interior of the system thus 

 modified became quite different. Under the action of the 

 preponderating mass of the sun (that of the rings was not 



