﻿Prof. Tyndall on Cometary Theory. 245 



shadow, it is only necessary to assume that the sun's calorific rays 

 are absorbed more copiously by the head and nucleus than the 

 actinic rays, This augments the relative superiority of the ac- 

 tinic rays behind the head and nucleus, and enables them to 

 bring down the cloud which constitutes the comet's tail. 



"5. The old tail, as it ceases to be screened by the nucleuses 

 dissipated by the solar heat ; but its dissipation is not instan- 

 taneous. The tail leans towards that portion of space last 

 quitted by the comet, a general fact of observation being thus 

 accounted for. 



" 6. In the struggle for mastery of the two classes of rays a 

 temporary advantage, owing to variations of density or some 

 other cause, may be gained by the actinic rays even in parts of the 

 cometary atmosphere which are unscreened by the nucleus. Oc- 

 casional lateral streamers, and the apparent emission of feeble 

 tails towards the sun would be thus accounted for, 



u 7. The shrinking of the head in the vicinity of the sun is 

 caused by the beating against it of the calorific waves, which dis- 

 sipate its attenuated fringe and cause its apparent contraction. 



" Throughout this theory 1 have dealt exclusively with true 

 causes, and no agency has been invoked which does not rest on 

 the sure basis either of observation or experiment. It remains 

 with you to say whether in venturing to enunciate it I have 

 transgressed the limits of ' rational speculation/ 



" If I have done so, surely I could not have come to a place 

 more certain to ensure my speedy correction. If the theory 

 be a mere figment of the mind, your Adams and your Stokes 

 (both happily here present), to whom I submit the speculation 

 with the view of having it instantly annihilated by astronomy 

 and physics, if it merit no better fate, will, I doubt not, effec- 

 tually discharge that duty, and thus save both you and me from 

 error before it has had time to lay any serious hold on our ima- 

 gination." 



The leniency with which this speculation was received at Cam- 

 bridge induces me to submit it to the critical judgment of the 

 readers of the Philosophical Magazine*. 

 I remain, Gentlemen, 



Royal Institution, Your obedient Servant, 



March 15, 1869. JOHN TyNDALL. 



* There may be comets whose vapour is un decomposable by the sun, 

 or which, if decomposed, is not precipitated. This view opens out the pos- 

 sibility of invisible comets wandering through space, perhaps sweeping 

 over the earth and affecting its sanitary condition without our being other- 

 wise conscious of their passage. As regards tenuity, I entertain a strong 

 persuasion that out of a few ounces (the possible weight assigned by Sir 

 John Herschel to certain comets) of iodide-of-allyl vapour, an actinic cloud 

 of the magnitude and luminousness of Ponati's comet might be manu- 

 factured. 



