Sir George Gabriel Stokes. 205 



less doubtful hypotheses as the distribution of matter in the interior 

 It was reserved for Stokes to point out that, in virtue of a simple 

 theorem relating to the potential, the law of gravity follows immediately 

 from the form of the surface, assumed to be one of equilibrium, and 

 that no conclusion can be drawn concerning the internal distribution of 

 attracting matter. 



From an early date he had interested himself in Optics, and 

 especially in the Wave Theory. Although, not long before, Herschel 

 had written ambiguously, and Brewster, the greatest living authority, 

 was distinctly hostile, the magnificent achievements of Fresnel had 

 converted the younger generation ; and, in his own University, Airy had 

 made important applications of the theory, e.g., to the explanation of 

 the rainbow, and to the diffraction of object-glasses. There is no sign 

 of any reserve in the attitude of Stokes. He threw himself without 

 misgiving into the discussion of outstanding difficulties, such as those 

 connected with the aberration of light, and by further investigations 

 succeeded in bringing new groups of phenomena within the scope of 

 the theory. 



An early example of the latter is the paper " On the Theory of 

 certain Bands seen in the Spectrum." These bands, now known after 

 the name of Talbot, are seen when a spectrum is viewed through an 

 aperture half covered by a thin plate of mica or glass. In Talbot's 

 view the bands are produced by the interference of the two beams which 

 traverse the two halves of the aperture, darkness resulting whenever the 

 relative retardation amounts to an odd number of half wave-lengths. 

 This explanation cannot be accepted as it stands, being open to the 

 same objection as Arago's theory of stellar scintillation. A body 

 emitting homogeneous light would not become invisible on merely 

 covering half the aperture of vision with a half wave-plate. That 

 Talbot's view is insufficient is proved by the remarkable observation 

 of Brewster that the bands are seen only when the retarding plate 

 is held towards the blue side of the spectrum. By Stokes' theory 

 this polarity is fully explained, and the formation of the bands is 

 shown to be connected with the limitation of the aperture, viz., to be 

 akin to the phenomena of diffraction. 



A little later we have an application of the general principle of 

 reversion to explain the perfect blackness of the central spot in 

 Newton's rings, which requires that when light passes from a second 

 medium to a first the coefficient of reflection shall be numerically the 

 same as when the propagation is in the opposite sense, but be affected 

 with the reverse sign the celebrated "loss of half an undulation." 

 The result is obtained by expressing the conditions that the refracted 

 and reflected rays, due to a given incident ray, shall on reversal 

 reproduce that ray and no other. 



