Remarks on the Tails of Comets. 61 



he know, though he may yet learn, the amount of his indebted- 

 ness to this ver3' medium for the transmission of all light. It is 

 generally known that the propagation of light has received two 

 different explanations. One by Newton, who supposed it to con- 

 sist of minute particles projected from all bodies by an inherent 

 force. Another by Huygens, the great Dutch astronomer, who 

 believed light to be transmitted to the optic nerves by vibrations, 

 communicated by luminous bodies to the particles of an ethereal 

 medium. The Newtonian theory, it has long been acknowledg- 

 ed, cannot be true, being entirely inadequate to an explanation of 

 a great variety of optical phenomena.* The Huygenian hypo- 

 thesis, somewhat analogous to the theory of sound, illustrates in 

 a satisfactory and beautiful manner, the general phenomena of 

 light, and its propagation ; and the experiments of Fresnel and 

 Young having rendered it nearly unquestionable, it has received 

 the sanction of the ablest philosophers of the present age.f Assum- 

 ing this theory as the true one, the ethereal medium, instead of 

 an impediment, is indispensable to the transmission of light. 

 Thus, in any view of the subject, this "great objection," as the 

 writer styles it, to make the most of it, may not be the greatest. 

 " We cannot suppose," continues the writer, " that if all the 

 light cast on a comet at that distance from the sun at which the 

 tail begins to be formed, was concentrated into one point, its in- 

 tensity would be nearly so great as that of the light received di- 

 rectly from the sun in the space immediately surrounding him. 

 If, therefore, the theory proposed were correct, we should expect 

 to see the sun enveloped in a luminous vapor which would extend 

 many thousands, if not millions of miles." To this I answer, 

 that neither the sun, nor the region of the sun, has ever been 

 seen under any material change of circumstances, to say nothing 

 of the zodiacal light, which it is to be hoped the writer has seen. 

 The light of a cornet's tail is not in itself a strong light ; its bril- 

 liancy, as I apprehend, is mainly to be attributed to the fact of its 

 shining in a region of extreme darkness ; for if an object, even of 

 dark color, retaining the ordinary light of the sun, could be pre- 

 sented to our view in the darkness of night, its brilliancy would 



* Young's Lectures, Vol. I. 



t In an address to the Astronomical Society of London, Sir J. F. W. Herschel 

 has the following remark: " Of the existence of such a fluid as the efficient cause 

 of liirht, we have demonstrable evidence." London Athenseum, for Feb. 1840. 



