Sir George Gabriel Stokes. 209 



consist chiefly of light polarised in the plane of reflection. On this 

 fact Stokes founded an important argument as to the direction of 

 vibration of polarised light. For "if the diameters of the (suspended) 

 particles be small compared with the length of a wave of light, it 

 seems plain that the vibrations in a reflected ray cannot be perpen- 

 dicular to the vibrations in the incident ray." From this it follows 

 that the direction of vibration must be perpendicular to the plane of 

 polarisation, as Fresnel supposed, and the test seems to be simpler and 

 more direct than the analogous test with light diffracted from a grating. 

 It should not be overlooked that the argument involves the supposition 

 that the effect of a particle is to load the aether. 



It was about this time that Lord Kelvin learned from Stokes 

 " Solar and Stellar Chemistry." " I used always to show [in lectures 

 at Glasgow] a spirit lamp flame with salt on it, behind a slit prolonging 

 the dark line D by bright continuation. I always gave your dynamical 

 explanation, always asserted that certainly there was sodium vapour 

 in the sun's atmosphere and in the atmospheres of stars which 

 show presence of the D's, and always pointed out that the way to find 

 other substances besides sodium in the sun and stars was to compare 

 bright lines produced by them in artificial flames with dark lines of 

 the spectra of the lights of the distant bodies."* 



Stokes always deprecated the ascription to him of much credit in 

 this matter ; but what is certain is, that had the scientific world been 

 acquainted with the correspondence of 1854, it could not have 

 greeted the early memoir of Kirchhoff (1859) as a new* revelation. 

 This correspondence will appear in Vol. IV of Stokes' Collected Papers, 

 now being prepared under the editorship of Prof. Larmor. The 

 following is from a letter of Kelvin, dated March 9, 1854 : "It was 

 Miller's experiment (which you told me about a long time ago) which 

 first convinced me that there must be a physical connection between 

 agency going on in and near the sun, and in the flame of a spirit lamp 

 with salt on it. I never doubted, after I learned Miller's experiment, 

 that there must be such a connection, nor can I conceive of any one 



knowing Miller's experiment and doubting If it could 



only be made out that the bright line D never occurs without soda, I 

 should consider it perfectly certain that there is soda or sodium in 

 some state in or about the sun. If bright lines in any other flames can 

 be traced, as perfectly as Miller did in his case, to agreement with 

 dark lines in the solar spectrum, the connection would be equally 

 certain, to my mind. I quite expect a qualitative analysis of the sun's 

 atmosphere by experiments like Miller's on other flames." 



By temperament, Stokes was over-cautious. " We must not go too 

 fast," he wrote. He felt doubts whether the effects might not be due 

 * Letter to Stokes, published in Edinbiirgli address, 1871. 



