in the Seventeenth Centnry. 11 



of Eobert Hooke (b. 1635, d. 1703), one of the founders of the 

 Eoyal Society, and at one time its Secretary. 



Hooke, who was both an observer and a theorist, made two 

 experimental discoveries which concern our present subject ; but 

 in both of these, as it appeared, he had been anticipated. The 

 first* was the observation of the iridescent colours which are 

 seen when light falls on a thin layer of air between two glass 

 plates or lenses, or on a thin film of any transparent substance. 

 These are generally known as the " colours of thin plates," or 

 " Newton's rings " ; they had been previously observed by Boyle.f 

 Hooke's second experimental discovery,^ made after the date of 

 the Micrographia, was that light in air is not propagated exactly 

 in straight lines, but that there is some illumination within the 

 geometrical shadow of an opaque body. This observation had 

 been published in 1665 in. a posthumous work of Francesco 

 Maria Grimaldi (b. 1618, d. 1663), who had given to the phe- 

 nomenon the name diffraction. 



Hooke's theoretical investigations on light were of great 

 importance, representing as they do the transition from the 

 Cartesian system to the fully developed theory of undulations. 

 He begins by attacking Descartes' proposition, that light is a 

 tendency to motion rather than an actual motion. " There is," 

 he observes, 1 1 " no luminous Body but has the parts of it in 

 motion more or less " ; and this motion is " exceeding quick." 

 Moreover, since some bodies (e.g. the diamond when rubbed or 

 heated in the dark) shine for a considerable time without being 

 wasted away, it follows that whatever is in motion is not per- 

 manently lost to the body, and therefore that the motion must 

 be of a to-and-fro or vibratory character. The amplitude of the 

 vibrations must be exceedingly small, since some luminous bodies 

 (e.g. the diamond again) are very hard, and so cannot yield or 

 bend to any sensible extent. 



* Micrographia, p. 47. t Boyle's Works (ed. 1772), i, p. 742. 



% Hooke's Posthumous Works, p. 186. 



Pkysico- Mathesis de lumine, coloribits, et iride. Bologna, 1665 ; book i, prop. i. 

 || Micrographia, p. 55. 



