124 Britain's Heritage of Science 



He wrote from the point of view of the elastic solid theory 

 of light, which now is abandoned, but his papers, and more 

 especially that on the Dynamical Theory of Diffraction, have 

 lost none of their value. 



Though a keen mathematician, Stokes was equally 

 interested in realities, and he has given us at least one 

 experimental discovery of primary importance. It was known 

 already to the Jesuit Kircher (1601-1680), and to Robert 

 Boyle, that extracts of certain woods presented a different 

 appearance when examined by transmitted or reflected light ; 

 John Herschel and David Brewster added some material 

 facts, and though they tried to theorize on them, they did 

 not make much headway in fitting the facts into the general 

 framework of Optics. Stokes attacked the problem in the 

 true Newtonian manner. Sunlight admitted through a slit 

 in a shutter entered the room, and, after passing through 

 three prisms, was made to form a spectrum on a screen. 

 Solutions of the substances to be examined, such as sulphate 

 of quinine or esculine, were placed in a test tube, and then 

 passed along the screen, so that they were successively 

 illuminated by the different colours of the spectrum. In the 

 red, yellow, green and blue, the substances behaved much 

 like transparent liquids, but when placed in violet they 

 began to shine, emitting a strong blue light, and this was 

 accentuated when the test tube was moved beyond the visible 

 spectrum, into what we now call the ultra-violet. The 

 existence of such rays had already been proved by means of 

 their chemical action, but Stokes widened their range to a 

 quite unexpected degree by using prisms made of quartz, 

 instead of glass; for the glass, as he showed, strongly 

 absorbed those rays. The practical application of these 

 researches, extending optical investigations into the regions 

 of waves which are too short to affect our eyes, became 

 apparent after the introduction of spectrum analysis, and 

 Stokes himself, in a subsequent research, investigated the 

 ultra-violet spectra of metals. But at the time, the novel 

 result emerging from the work was the discovery that the 

 substances experimented upon had the power of changing 

 the wave-length of the light which fell upon them. This was 

 quite contrary to what Newton had taught. Newton was 



