8o Expeiimtnls and Obfervdtions an Light. 



exhibited iwi flreamS of faint light fliooting out both ways from the beam into the 

 fhadow like the tails of comets, at an angle of about ten or twelve degrees, and fometimes 

 farther; at the fame time that a line of light appeared at the edge of the knife, vifiblc even 

 out of the direftion of the general light from parts beyond either the point or handle of 

 the knife. This light, Newton obferves, was contiguous to the edge, and narrower than 

 the light of the mnermojl fringe, and pafled between it and the knife. The author, in his 

 obfcrvation J, remarks, that the preceding circumftances conllitute only one of the 

 many cafes of the inflexions of light in which no fringes are formed, but only a general 

 derivation and divergence of the rays is produced, which is owing'to the parallelifm of 

 the rays of light not being changed by their paflage through the large hole, but wherever 

 the diverging of a beam of light is fuch that the inflexions of a body can give them 

 the due arrangement, fringes are formed of endlefs varieties as thefe circumftances vary. 

 Three cafes of fringes, making, with the former, four general cafes of inflexions, arc 

 then ftated as including thefe varieties of appearance as the difperfion of the beam from 

 its greatefl, is diminifhed to its fmalleft ftate of divergency and to parallelifm, and it is 

 explained and ftiown by figures how the formation of fringes depends upon the divergency, 

 and is deftroyed by the parallelifm of a beam. The Newtonian experiment is then re- 

 peated with a beam of light pafled through a hole quarter of an inch wide, in a plate 

 of lead, the two ftreams of light obferved by him concerning the origin and exiftence of 

 which much doubt and difficulty had arifen among philofophers, are (hown to be owing 

 the one to the edge of the lead of the hole, the other to the edge of the knife, and to be 

 produced in the parallel beam of light. The line of light obferved by Newton is noticed 

 as indeed worthy of particular attention, being in a very remarkable manner derived out 

 of the whole body of the light pafling by the edge, and not merely from that contiguous to it, 

 and not being narrower than the inmojl fringe, or pafling within it, bccaufe in this cafe no 

 fringes are or can be formed, pafling, however, at thofe very points near the body at 

 which, under other circumftances, the many neareft fringes are formed. 



According to obfervations the fixth and feventh, by approaching the parallel edge of 

 another knife to that of the former, at the diftancc of the 400th part of an inch, juft 

 before they touch, the whole pafling light is divided into two ftreams, each bent towards 

 the neareft edge with a dark fhadow between them, which ftreams vanifti upon contact of 

 the knives, their parts farthe/l from the direft light vanifliing /«/?. As the knives, however, 

 approached each other, and before the fliadow appeared, the three fringes themfelves ap- 

 peared on the inner ends of the ftreams on either fide of the dired light, and grew dif- 

 tinfter and larger until they a/l vaniflied, the outmoft firft, the middlemoft next, and 

 inmoft laft, leaving the ftreams of light defcribed in the fifth and fixth obfervations, Thefe 

 ftreams,, therefore, Newton concluded pafled by the edges at le/s diftances than any of 

 the fringes, and the fringes at different diftances according to the order of their 

 vaniftiing. 



Obfcrvation 



