center is like a large globular nebula with a very marked condensation, whence radiate branches arranged in the form of spirals. In several points of these branches other centers of condensation are noticed. Sir John Herschel had classed this among the nebulæ of rounded, globular form, doubtless because the central nebulosity was the only one revealed by his telescope." The formation of the sub-centers in this nebula (which is between the Great Bear and Boötes) should be particularly noted in connection with the coalescence of planets as above described. In a note to Guillemin's work, Professor Lockyer says, "The proper motion of nebulæ has not yet been inquired into, because everybody, looking upon them as irresolvable star-clusters, thought them infinitely remote. Now, however, that we know they are not clusters of stars, properly so called, it is possible that they may be much nearer to us than we imagine." In connection with the double-sun spiral nebula shown in the preceding illustration, Guillemin says, "We have noticed nebulæ accompanied by systems of double or multiple stars, placed in a manner so symmetrical in the midst of the nebulosity that it is impossible to doubt the existence of a real connection between the stars and the nebulæ." And Flammarion says of these apparently globular nebulæ, when under the observation of more powerful telescopes, "In the place where pale and whitish clouds gave out a calm and uniform light, the giant eye of the telescope has discerned alternately dark and luminous regions,"--that is to say, they reveal the operation of the opposite forces of attraction and repulsion, and are spiral. While gaseous nebulæ may be of any conceivable form, the direction and operation of the forces which will determine their character as solar systems must be similar, just as with the forms of organic life, and the only nebulæ which reveal a distinct systematic development in harmony with a working solar system are the spiral. There is no difficulty whatever in tracing such a nebula through all its formative stages, as we have done, and we can, in fact, see painted on the background of the sky every step of the shifting tableau through which such forms must pass.