596 Dr. G; Johnstone Stoney on 



the lens is supposed to consist, is made a medium which 

 extends through all space. 



50. Let us suppose P x to be placed at the point in space 

 which is represented by the centre of the accompanying figure. 



Fig. is:' 



Diagram of distances in space which may be immense *. 



Round P,, which is on the surface of the glass lens, draw the 

 Sphere rf of such a size that the lens lies completely inside it. 

 Let us next imagine the transparent glass to be extended 

 beyond the lens so as to become a uniform medium filling all 

 space. It will be convenient to suppose that the lens lies to 

 the right-hand side of the centre, in which case the light we 

 have to deal with will be the light emitted by P 2 towards the 

 east or right-hand side of the diagram. The part of this 

 light which is of wave-length X, is an undulation of hemi- 

 spherical waves travelling from the centre of the figure towards 

 the east or right-hand side. We must recognize P x as a 

 source of energy external to the medium ; and accordingly 

 the motion of the medium in its immediate neighbourhood 

 will be of the kind which we have designated turmoil. But 

 if we describe round Pj a tiny sphere, not shown in the 



* In this diagram P the punctum emitting light, is supposed to be 

 situated at the centre of the spheres. It is surrounded by a tiny sphere p 

 («. e. with radius p) which is too small to be represented in the figure. 

 The spheres M 15 M 2 , and M 3 have radii which equal the distances to 

 which light travels in the medium in the periods t, 2t, and 3r 

 respectively. 



t Sphere r means the sphere which has r as radius. 



