BY G. R. SMALLEY, ESQ. 337 



There is another theory propounded. From observations of 

 Halley's Comet especially, it is supposed that comets possess the 

 property of throwing off and leaving behind them, in a vaporous 

 state, some portion of their matter, which is repelled by the sun ; 

 whilst the remaining portion is still attracted by the sun, and 

 proceeds under the usual law of gravitation. This theory would 

 certainly explain the phenomenon of Encke's Comet for if some 

 portion of the mass were entirely disposed of, the remainder, 

 having less mass than before, would be more subject to the sun's 

 attraction : consequently, the orbit would be diminished, and 

 therefore the periodic time continually decreased. Now without 

 pausing to reflect upon the complicated nature of the theory thus 

 offered, we may observe that the phenomena on which it in a 

 great measure depends, have not been witnessed in many comets, 

 nor do the vaporous effusions, even in Halley's Comet, appear to 

 have been alvjays in the direction where it would meet with least 

 resistance. When two theories, equally probable, are proposed 

 for our acceptance, we are naturally prepared to receive the more 

 simple. Of the simplicity of Encke's theory compared with the 

 latter, there can be, I imagine, but little question. Again, are 

 they both equally possible ? The second may indeed be quite 

 possible, so far as we know, but I am not aware of any evidence 

 that renders such a conclusion decisive ; and with regard to 

 Encke's theory, it is so easily reducible to ordinary dynamical 

 principles, that one question only can be raised, and that is, 

 " the probable existence of an etherial medium." 



There are independent proofs of this, and it renders Encke's 

 theory in the highest degree probable. 



Formerly, light was supposed to be emitted from self-luminous 

 bodies, in extremely minute particles, travelling with enormous 

 velocity, and impinging on the retina. This " Theory of 

 Emanations " sufficiently accounted for most optical phenomena. 



It failed, however, to explain the experiment of Newton's 

 Rings, in which, if a lens of ever so slight a curvature be pressed 

 upon a piece of plate glass, concentric rings of different colours 

 are observed, separated by dark intervals. 



But this singular phenomenon is fully accounted for by 

 " The Undulatory Theory of Light" a theory whose truth has been 



