226 



HISTORY OF SCIENCE. 



light to pass in straight lines. The shadows in this case were sur- 

 rounded by three coloured fringes, and within the shadow itself similar 

 fringes might be detected. When light was allowed to stream in 

 through two small apertures or pin-holes placed so near to each other 

 that the cones of light intersected each other, Grimaldi observed that 

 a spot common to the circumference of each was darker than the same 

 spot illumined by either of the lines separately, and he announced the 

 fact in this remarkable proposition : "A body actually illuminated may 

 be darkened by adding a light to that which it already receives." Such 

 were the first observations of the remarkable phenomena of diffraction 

 and interference. 



In England Dr. Hooke occupied himself with experiments on this 

 subject without knowing what had already been done by the Italian 

 philosopher, and in 1672 he announced to the Royal Society that he 

 had in preparation a paper " containing the discovery of a new pro- 

 perty of light not mentioned by any previous writer on optics." Newton 

 afterwards took up this subject, and he began by making accurate 

 measurements of the diameter of a hair and of the breadth of the sha- 

 dows, and he found that the breadth of the shadow was indeed greater 

 than the proportional distance of the aperture from the hair. He ex- 

 plained the facts by supposing that rays passing near the hair are turned 



FIG. 



aside as if by a repulsive force. Thus H, Fig. no, represents a sec- 

 tion of the hair ; and abed rays passing at different distances, the 

 ray a a will be more deflected than bb,bb than cc, and so on ; the result 

 would be a crossing of the rays in such a manner that the shadow 

 would be outwardly convex. Newton gave to this class of phenomena 

 the name of inflexion. The phenomena of inflexion received, as we 

 shall find, quite another explanation from Young and Fresnel. 



The undulatory theory of light was first put forth by Huyghens in 

 a communication addressed to the Academy of Sciences at Paris, in 

 1678; but afterwards he made this theory the subject of a work en- 

 titled, " Traite de la Lumiere" published in 1690. The original idea 

 of Huyghens was that an inconceivably subtile and highly elastic me- 



