Three years before Chamberlin and Moulton advanced their planetesimal theory, I had speculated as to the possibility of tidal forces breaking up a star, and generating a solar system. In 1916, I investigated mathematically what would actually happen when one star raised violent tidal forces on another. The results I obtained seemed to me to demolish the planetesimal theory of Chamberlin and Moulton, and led me to put forward the present-day “Tidal Theory,” which I believe a large proportion of astronomers now accept as giving the _most probable_ origin of the solar system; it can of course make no claim to finality or certainty. TIDAL THEORY. When two stars or other bodies pass close to one another without collision, the primary effect must be that each raises tides in the other. The closer the approach, the higher the tides in general, although something must depend also on the speed with which the bodies pass one another, because this determines the length of time during which they influence one another. It is likely that the two spiral arms which give their name and characteristic appearance to the spiral nebulae may owe their inception to a somewhat similar tidal action. Conditions here are different in that the rotation of the nebulae in any case causes them to emit matter in their equatorial planes, so that even small tidal forces should then cause this matter to concentrate in two symmetrical arms. Under stellar conditions a far closer approach is necessary to draw matter out from the star, and it is then most likely that there will be two unequal and dissimilar arms, or possibly only one arm. [Illustration: PLATE XXII _Mt Wilson Observatory_ Two Nebulae (N.G.C. 4395, 4401) suggestive of Tidal Action] [Illustration: PLATE XXIII The twin Nebulae N.G.C. 4567-8] [Illustration: _Mt Wilson Observatory_ The Nebula N.G.C. 7479] If the approach is very close indeed, the tides may assume an entirely different aspect from the feeble tides which the sun and moon raise in our oceans; they may take the exaggerated forms of high mountains of matter moving over the surface of the star. An even closer approach may transform these mountains into long arms of gas drawn out from the body of the star. If, as will generally be the case, the two stars are of unequal weights, the lesser will in general suffer more disturbance than the weightier.