on Dr. Faraday's Paper on Ray-vibrations. 533 



more especially of light, by supposing that when there is no 

 body obviously occupying the path of the light, &c, the vibra- 

 tions which are assumed as the foundation of the undulations 

 producing the phsenomena are transmitted on the lines of force 

 by what (for want of a received term) may be called lateral 

 shakes. 



2. The possibility of removing the idea of substance and 

 substituting for it that of centres of force. 



I shall treat of these in the order in which I have written 

 them above. 



1. With regard to the transmission of light through the 

 planetary spaces. 



Dr. Faraday and myself agree in receiving the undulatory 

 theory of light with transversal vibrations, as applicable to 

 those phaenomena which present themselves in ordinary optical 

 experiments. Without any wish therefore to dogmatize on 

 this matter, I shall assume the undulatory theory in all the 

 following remarks. 



It is admitted that vibrations forming progressive undula- 

 tions are required for the explanation of certain crystalline 

 and other phaenomena. But I must claim somewhat more. 

 Progressive undulations (leaving the nature of their vibrations 

 undetermined) are required to explain the phaenomena of dif- 

 fraction', and these progressive undulations must not be of 

 the nature of radial shakes, where each shake derives its 

 virtue or existence from the momentary influence of the dis- 

 tant origin, but they must be true waves, of which the mecha- 

 nical characteristic is that the motion of a succeeding set of 

 particles is determined by the relative motion of the prece- 

 ding set of particles; the order of "preceding" and "suc- 

 ceeding" not being confined to a radial line or to any lines 

 whatever, but being such that the motion of particles may be 

 origin of motion to other particles extending round them 

 through a very large solid angle. I defy any one to put to- 

 gether a theory of radial lines subject to lateral shakes which 

 shall explain diffraction ; and I say that it will be found ab- 

 solutely necessary to admit, in the theory explanatory of dif- 

 fraction, that each disturbance of particles produces a swell 

 (to use language derived from the motion of water), which 

 swell is propagated in all directions through at least a very 

 large solid angle. Now the consequences of this are very 

 important. Diffraction takes place in air; therefore the vi- 

 brating medium exists in air, and the undulations are trans- 

 mitted by it, and not by radial shakes. As far as we can per- 

 ceive air in its utmost degree of tenuity, it produces refac- 

 tion-, refraction inexorably requires for its explanation a 



