353 Prof. Challis on the Resistance of the Luminiferous 



Group III. 



Ratio of axes of thermal ellipse 1 to 1*50. Mean ratio of deflection 

 1 to 9-5. 



Apricot. 

 Pistachio. 

 Siberian Acacia. 



Pernambuco red wood. 



Japan red wood. 



Puerto- Caballo yellow wood. 



Group IV. 



Relation of the a.\es of the thermal ellipses 1 to TS. Mean ratio 

 of deflection 1 to 14-0. 



Wevmouth fir. 



Willow (two examples). 



Chestnut (three examples) 



Lime. 



Alder. 



Birch. 



Poplar (three examples). 



Aspen. 



Pine. 



Fir. 



Magnolia. 



Iron wood. 



Tamarind. 



Palmassu. 



" Kistenholz." 



Caoba (Havannah Cedar). 



Savanilla yellow wood. 



LVII. On the Resistance of the Luminiferous Medium to the 

 Motions of Planets and Comets. By Professor Challis*. 



KEEPING still in view the investigation of a mathematical 

 theory of physical forces on the hypothesis of the dyna- 

 mical action of a continuous elastic fluid pervading space, I pro- 

 pose, as a preliminary step, to give reasons for assuming the 

 existence of such a medium, and to endeavour to obtain as di- 

 stinct an idea as possible of its nature and properties. As this 

 inquiry is closely connected with the undulatory theory of light, 

 the principles of that theory, and the indications it gives of the 

 existence and qualities of the medium by which light is supposed 

 to be transmitted through space, necessarily come under consi- 

 deration. But besides that the medium must be such as to 

 account for the various phrenomena of light, it must also be con- 

 sistent with other classes of facts to which it can be shown to be 

 related. For instance, since physical astronomy has proved that 

 the planets are not perceptibly acted upon by any other force 

 than gravitation, we must show that the luminiferous medium 

 offers no sensible resistance to their motions. To reconcile this 

 fact with the properties of a medium, which has been supposed 

 to give rise to the phajnomena of light by the motions of its 

 separate atoms, is acknowledged to be a difficulty by the advo- 

 cates of that kind of agency. In a connnunicatiou to the Philo- 



* Communicated by the Author. 



