50 LIFE AND WORK OF SIR JAGADIS C. BOSE 



them. Still, with the reasoned certitude of his mathematical 

 treatment, Maxwell stuck to it that the currents are none 

 the less there ; and so framed his electro-magnetic theory 

 of light. For now, from this point of view, the light- waves 

 of the ether, already lucidly but separately visualised 

 and measured by Fresnel and others, may be interpreted 

 as the product of rapidly alternating currents set up in the 

 dielectric ether (and as it were the oscillations of the elastic 

 springs) and thus carried through space. The mathe- 

 matical mind was impressed by Maxwell's theory and its 

 calculations ; but neither physicist nor plain man could 

 be satisfied without concrete proof, through experimental 

 demonstration. But how reach experimental mastery 

 and understanding of alternating currents and oscillating 

 discharges of such high frequency as is required by 

 the known velocity of light about 300,000 kilometres 

 (186,000 miles) per second ? And with -the numberless 

 waves in that second, when even the longest visible red rays 

 are pouring upon our retina every second at the rate of at 

 least 25,000 crowded into every inch of that vast distance ; 

 and those which affect the photographic plate are more 

 than twice as many in the same time ? The difficulty of 

 experiment is here obvious. Still, experimenters set to 

 work ; and Feddersen, working with the Ley den jar, 

 photographed its long-known spark, by help of a rapidly 

 revolving mirror. Now if this discharge be a continuous 

 one, the photograph would be that of a luminous streak, 

 like that of a star slowly photographed while the earth turns 

 round. But the photographs showed successive firefly- 

 like flashes, proving the intermittency of the discharge, 

 and the photographs of sparks showed these as not homo- 

 geneous, but as symmetrically contrasted, the bright points 

 at one end corresponding to dark points at the other, and 

 conversely. Here, then, was clear ocular demonstration 

 that the discharge, which to our eye seems but a 

 single and instantaneous spark, is really a succession of 

 sparks, in oscillation between positive and negative. This 



