ROBERT HOCKK ON ELECTRIC LIGHT. 431 



been the first recognition of the electric flash the spark 

 in contradistinction to the glow. This was also the first 

 attempt to explain light electrically caused, as being in 

 common with all light a "peculiar kind of internal motion 

 of the particles of a body," but specifically due to the 

 nature of the body itself, the mode of exciting the motion 

 (rubbing) and the degree of excitation produced. More- 

 over, he went further and asserted that there is "an inter- 

 nal vibrative motion of the parts of the electric bodies, and 

 so soon as ever that motion ceases, the electricity also 

 ceases " so that, not only did he find the particular mani- 

 festation of electricity as light due to vibration, but as- 

 cribed the entire electrical phenomenon, even when 

 appearing as attraction, to the same cause. 



Hooke's theory of light, following substantially that of 

 Descartes, and involving the assumption that space is 

 filled with something that transmits light instantaneously, 

 was overthrown by Roemer's observations of Jupiter's / 

 moons in 1676, resulting in a determination of the velocity 

 of light. 1 Then came Newton's emission theory, which 

 yielded to the now-accepted undulatory hypothesis of 

 Young; but, none the less, such concepts of the electrical 

 phenomena as Hooke made were a long way ahead of the 

 " unctuous steams" and "rebounding effluvia" which had 

 preceded them.* 



So far as is known, Hooke made no electrical or mag- 

 netic discovery of major importance. The catholicity of 

 his work was against his doing so. It is seldom that the 

 inventor who expends his energy in an infinitude of de- 

 tails ever leaves behind him any one great monumental 

 achievement. There is an apparent gap between the end- 

 less mechanical refinements of Hooke's multitudinous in- 

 struments and his dynamical theories of heat, .light and 

 electricity, which it seems should be filled by tangible ac- 

 complishments of a higher order than the former. If he 

 did so, he concealed them; and again revealed another one 



*Tyndall: On Light. London, 1875, 45. 



