THE EVOLUTION OF SATELLITES. 123 



The fact that two stars are now found to be revolving about each 

 other leads to the conviction that their relationship is not a casual one, 

 but that they have been connected from an early epoch, which for 

 convenience we may call the origin of the system. It appears almost 

 beyond question that this starting point must have been at a time 

 when the two stars were united in a single rotating mass. As the basis 

 of his explanation of the manner in which a single mass may split into 

 two, Mr. See takes certain theoretical investigations as to the shapes 

 which a mass of gravitating and rotating fluid is capable of main- 

 taining. I will not recapitulate his theories, but I wish to emphasize 

 the uncertainties with which we are here brought face to face. 



Many years ago Sir John Herschel drew a number of twin nebuhe 

 as they appear through a powerful telescope. The drawings probably 

 possess the highest degree of accuracy attainable by this method of 

 delineation, and the shapes jnesent evidence confirmatory of Mr. See's 

 theory of the fission of nebulae. But since Herschel's time it has been 

 discovered that many details, to which our eyes must remain forever 

 blind, are revealed by celestial photography. The photographic film 

 is, in fact, sensitive to those photographic rays which we may call 

 invisible light, and many nebulfe are now found to be hardly recogniz- 

 able, when photographs of them are compared with drawings. A 

 conspicuous example of this is furnished by the great nebula in Andro- 

 meda ,-^ for whereas the drawing exhibits a cloud with a few dark 

 streaks in it, the photograph shows a flattened disk surrounding a 

 central condensation; moreover, the disk is seen to be divided into 

 rings, so that the whole system might have been drawn by Laplace to 

 illustrate his celebrated nebular hypothesis of the origin of the solar 

 system. 



Photographs, however, do not always aid interpretation, for there 

 are some which serve only to increase the chaos visible with the tele- 

 scope. We may suspect, in fact, that the complete system of a nebula 

 often contains masses of cool and photographically invisible gas, and 

 in such cases it would seem that the true nature of the whole will be 

 forever con{;ealed from us. 



Another group of strange celestial objects is that of the spiral neb- 

 ultB, whose forms irresistibly suggest violent whirlpools of incandescent 

 gas. Although in all probability the motion of the gas is very rapid, 

 yet no change of form has been detected. We are here reminded of a 

 rapid stream rushing past a post, where the form of the surface 

 remains constant whilst the water itself is in rapid movement, and it 

 seems reasonable to suppose that in these nebulai it is only the lines of 

 flow of the gas which are visible. Again, there are other cases in 

 which the telescopic view may be almost deceptive in its physical sug- 

 gestions. Thus, the Dumb-Bell JSTebula (27 Messier Yulpeculte), as 

 viewed telescopically, might be taken as a good illustration of a nebula 

 almost ready to split into two stars. If this were so, the rotation 

 would be about an axis at right angles to the length of the nebula. 



