Experimenti and Obfervations on Light. 79 



and the principle of attraftion is fo well eftabliflied, as to leave no doubt of its affording 

 the proper explanation of thefe appearances. 



In oppofition to the authority of Newton, he is quoted againft himfelf. Five paflages 

 are produced from his works, which prove, that on different occafions he reafoned incon- 

 Cftently and contradi£loriIy concerning thefe phenomena. 



This obfervation is concluded with the curious remark, that ftridlly fpeaking, all this 

 reafoning has been applied, to what in fa£t is not a (hadow, formed by the interception of 

 light by the body, as appears in a fubfequent obfervation y, but a dark interval of the 

 nature of thofe obferved between the fringes of colours afterwards mentioned. So little 

 have thefe phenomena been underftood ! 



The fecond and third obfervations of Newton next follow, and are fucceeded by the 

 author's obfervation /3, in which, inftead of only three noticed by Newton at the termi- 

 nation of the ftiadow of the hair, an uninterrupted fucceffion of fringes are defcribed, the 

 colours of which are dated to vary fomewhat from thofe, the fringes themfelves admitting 

 of various changes, and this new arrangement being made with a view to analogies, that 

 are ftated to exift between thefe and other fimilar appearances, and in order that they may 

 generally accord therewith, and with other obfervations of Newton made elfewhere. 



Newton's fourth obfervation defcribes the fucceffive appearance of the three fringes, 

 with their dark intervals at different diftances from the hair, and concludes from meafures 

 taken, that the fquares of the breadths of the fringes, and their intervals together, are in 

 the continual progreffion i, 4» t> *' t? °' thereabouts. Obfervation y, ftates, that at a 

 very fmall diftance from the hair, the fliadow appears diftindt, well-defined, and intenfely 

 black ; that at a greater diftance it is divided through its whole length by a line of light, 

 and changed to a double fhadow, refembling the fliadows of two hairs, but not fo intenfely 

 black as before ; and that at yet greater diftances the fhadow increafes in breadth, and 

 diminifties in blacknefs, whilft the line of light putting on colours, and becoming more 

 and more dilute, fpreads over and difappears within the fhadow, which again lofes its 

 double appearance. The firft only is a proper (hadow, which is quickly invaded and de- 

 ftroyed by the infle£led lights of the two neareft fringes on both fides of the hair, which 

 meet together at that diftance therefrom, and the apparent double fhadow confifts of the 

 two fucceeding dark intervals, which become fingle as the intermediate light becomps di- 

 lute, and ceafes to appear. This is what has been called a fliadow by Grimaldi and 

 Newton, and has been confidered and meafured as fuch. 



The fringes and their intervals, as they arc formed under various circumftances, and 

 their various dimenfions are then fhortly noticed, and it is affirmed, contrary to the ob- 

 fervation of Newton, that the breadths of the fringes and intervals, fo far from obferving 

 generally the proportions alEgned by him, do vary indefinitely through almoft all degrees- 

 of their poffible extenfion. 



According to the fifth obfervation of Newton, a beam of light pafling through a hole 

 \ of an inch wide by the edge of a knife, on the blade of which part of the light fell,, 



exhibited^ 



