7^ Experiments and Olfervaliont on Light. 



VIL 



Account of a Work entitled " Tke Obfervations of Newtok concerning the InJIeSions of 

 Light ; accompanied by other Obfervations differing from his ; and appearing to lead to a 

 change of his Theory of Light and Colours." By a Correfpondent. 



HIS anonymous work confifts of the obfervations of Newton concerning the inflec- 

 ticJPfc of light contained in his third Book of Optics, reprinted, and accompanied by new 

 fads and obfervations * made concerning the fame phsenomena, together with inferences, 

 which differ altogether from thofe of Newtoii. 



In his firft obfervation, Newton, repeating an experiment of Grimaldi, perceived as he 

 had done, that in a beam of folar light pafling through a fmall hole ~ of an inch wide, 

 the breadth of the fiiadow of a hair obferved at a diftance from the hole, was confiderably 

 greater than that of the hair itfelf. This he conceived could only be efFe£led by a repufftve 

 power in the hair upon the parallel rays of the beam, turning the light afide from the 

 fljadow, and rendering the fliadow confiderably broader than it ought to be, if the rays pafled 

 by in right lineji. By actual- meafurement he further afcertained, that the (hadow was 

 broader in proportion to the diftance from the hair when nearer, than when farther from 

 it ; and this he imagined to depend upon the diminution of the repulfive power upon the 

 more diftant rays of light, in confequence of which they were lefs turned afide from, 

 and therefore in their progrefs invaded and reduced the dimenfions of the Ihadow. ^ 



This of Newton is followed by an obfervation marked «, of the fame appearances. 

 In which the firft thing noticed is, that the beam of light after paflage through the fmall 

 hole, is no longer a beam of parallel rays, as Newton fuppofed, and has drawn it in his 

 figure, but confifts of divergent rays. This is proved to be the cafe by aftual meafure- 

 ment, due allowance being made for the penumbral light of the fun, and is ftated to ap- 

 pear more clearly by immediate infpeftion of the various changes produced in the beam. 

 This divergence is afcribed to the well eftabliflied principle of an attradlive force exercifed 

 by bodies upon light, in confequence of which when the hole is very fmall, its edges aft 

 upon the whole of the pafling light, and render it altogether divergent. 



In fuch divergent light, therefore, the fhadows of bodies ought to be of dimenfions con- 

 fiderably greater than the bodies themfelves,, m proportion to the divergence of the rays, 

 and to the diftance of obfervation. This proportion, however, is not exadlly preferved, 

 becaufe the rays in pafFing by the hair, undergo from the atlraElion of the hair a change of 

 dire£lion towards the (hadow, in confequence of which the fhadow is really lefs broad, than 

 it would have been if the rays had paffed in right lines, and this proportion of the fhadow 

 is greater at fmaller, than at larger diftances. Thefe things are clearly (hewn by a figure, 



• The CljferTations of the author are defignated by the letters a, |S, y, S, &c. of the Greek alphabet. — N. 



and 



