158 Dr. G-. Johnstone Stoney on Mr. Leivis Wright's 



by itself without its undergoing change as it advances. The 

 additional information which I have referred to above as being 

 supplied by the second mode of proof is that these trains of 

 plane wavelets advance in all the directions towards which the 

 spherical waves travel, and that their intensities, though of 

 infinitesimal amount, are proportional to the intensities of the 

 light carried in their respective directions by the spherical 

 waves. This enables us to trace the distribution of energy 

 among the plane wavelet components into which a system of 

 spherical waves surrounding a punctum may be resolved ; and 

 the knowledge of this distribution is for some purposes of much 

 use to the plrysicist. 



This resolution of a concentric system of spherical waves 

 into innumerable plane wavelets furnishes a correct resolution, 

 either for the whole of space or for any portion of space cut off, 

 not physically but only geometrically, by a closed surface of 

 any form, size, and position that may be desired ; as, for 

 example, for the space within the thickness of a cover-glass 

 (see footnote, p. 157), which we are, for the purposes of this 

 resolution, to picture to ourselves as a portion of an infinite 

 extent of glass. Moreover, it is an important corollary from 

 the foregoing resolution of the undulation surrounding a point 

 that any undulation whatever, however complex, in any portion 

 of the sether, using the word undulation in the sense above 

 denned, may be resolved into, or in other words is the same 

 physical event as, the coexistence of innumerable trains of 

 plane wavelets traversing that space in all the directions in 

 which light advances across it. 



This, which is the theorem enunciated on p. 335 of my 

 first paper, is a theorem intimately related to the familiar 

 geometrical axiom that a given surface may be resolved into 

 — i. e. is identical with the coexistence of — its innume- 

 rable so-called points ; each of these points being of in- 

 finitesimal extent when compared with the whole area of 

 the surface. This relation is more than an analogy ; an 

 actual physical relationship may be traced. Let us, for 

 example, consider the light which advances over the interval 

 between the objective field on the stage of a microscope 

 and the front lens of its objective. This light may be 

 resolved into innumerable plane-wavelet components tra- 

 versing the same space in all the directions towards which 

 any of the light shines, and extending laterally without limit 

 beyond the portion of space which lies between the object and 

 the objective. Now, assuming the objective to be optically 

 perfect, it can be proved that image x (that luminous image 

 near the back lens of the objective which is seen on removing 



