178 Mr KELLAND, ON THE DISPERSION OF LIGHT, 



fied: this requires the further condition, that 6 shall be much less in 

 vacuum than in refracting media. 



We have, then, clearly been led to the conclusion, not only that 

 the density of the aether is greater in vacuum than in refracting media, 

 but also that its particles act on each other with forces varying 171. 

 versely as the square of the distance. 



But I do not stop here ; it is a remarkable fact, and one which 

 demands particular attention, that various phenomena appear to indicate 

 not merely that the motion is in general transversal, but that it is al- 

 together so. 



Let us consider this point a little more accurately. Suppose the 

 forces which the particles of the medium exert to be repulsive, as 

 those of air, from which arise the phenomena of sound. A series of 

 particles constituting any vertical line being simultaneously impelled in 

 a horizontal direction would, by virtue of their repulsion, cause a similar 

 motion in those immediately in front of them, Avhilst the latter particles 

 would tend to check the impetus of the former, and thus vibrations 

 in the direction of transmission are simple to conceive, and easy to 

 explain. 



On the other hand were the particles attractive, no such motion 

 would be possible, except under peculiar restrictions. 



But suppose, notwithstanding, that the forces which the particles 

 exert are attractive — Let the system of particles in a vertical line have 

 a vertical motion, and the slightest consideration will shew us that the 

 immediate consequence is the production of a vertical motion in the 

 particles immediately in advance of them ; whilst, as before, the reci- 

 procal action of the latter particles tends to impede the motion of the 

 former. Here, then, we have as clear a case as before, and our general 

 conclusion from the whole is, that repulsive forces allow of direct, at- 

 tractive, of transversal vibrations only. In the former case I would 

 refer for a simple conception to the wind blowing over a field of corn, 

 and to a rope held between two persons, and jerked by one of them ; 

 in the latter, as they illustrate both the mode of vibration and the 

 action of the forces. 



