Medium to the Motions of Planets and Comets. 353 



sophical Magazine for December 1853 (p. 404), Mr. Rankine 

 makes the following statement : — 



" It has hitherto been always assnraed that the kind of motion 

 which constitutes light is a vibration from side to side, transmitted 

 trom particle to particle of the luminiferous medium, by means 

 ot torces acting between the particles. In order to account for 

 the transmission of such transverse vibrations, the luminiferous 

 medium has been supposed to possess a kind of elasticity which 

 resists distortion of its parts, like that of an elastic solid; and 

 in order to account for the non-appearance in ordinary cases of 

 eflects which can be ascribed to longitudinal vibrations, it has 

 been necessary to suppose farther, that this medium resists com- 

 pression with an elasticity immensely greater than that with 

 which It resists distortion; the latter species of elasticity bein- 

 nevertheless, sufficiently great to transmit one of the most pow' 

 ertul kinds of physical energy through interstellar space with a 

 speed in comparison with which that of the swiftest planets of 

 our system m their orbits is appreciable, but no more. 



*at seems impossible to reconcile these suppositions with the 

 tact, that the luminiferous medium in interstellar space oflFers no 

 sensible resistance to the motions of the heavenly bodies " 



It seems also inconceivable that a definite and permanent 

 arrangement of the ^therial atoms, which is an essential condi- 

 tion of the only «;,n-on- investigation of transverse vibrations 

 which the supporters of these views have given, can consist with 

 the tree motion of the heavenly bodies through the jether To 

 meet these difficulties, the author of the above extract proposes 

 an hypothesis of oscillations, which will be best stated in his 

 own words— "The hypothesis now to be proposed as a ground- 

 work for the unculatory theory of light, consists mainly m con- 

 ceiving that the luminiferous medium is constituted of detached 

 atoms or nuclei, distributed throughout all space, and endowed 

 with a peculiar species of polarity, in virtue of which three or- 

 thogonal axes m each atom tend to place themselves parallel 

 respectively to the corresponding axes in every other atom ; and 

 that plane-polarized light consists in a small oscillatory move- 

 ment of each atom round an axis transverse to the direction of 

 propagation."-(P.40G) "It is evident that how powerful 

 soever the polarity may be, which is here ascribed to the atoms 

 of the luminiferous medium, it is a kind of force which must be 

 absolutely destitute of direct influence on resistance to chan-e 

 of vo ume or change of figure in the parts of that mediun., or of 

 any body of winch that medium may form a part; and that, 

 consc(juentIy, the difficulty which in the hypothesis of vibrations 

 arises rom the necessity of ascribing to (he luminiferous medium 

 properties like those of an clastic solid, has no existence in the 



