294 Experiment of Newton. [Book III. 



a body, fhould pafs fo near to it as to be .within the 

 fphere of that power which the body pofiefles, it muft 

 necelTarily fuffer a change in its direction. Actual ex- 

 periments confirm the truth of ^his pofition, and to the 

 change in the direction of a particle of light, owing to 

 its nearnefs to a body, we give the name of inflec- 

 tion. 



From one of thefe experiments, made by Sir Ifaac 

 Newton, the whole of this fubject will be eafily im- 

 derftood. At the diftance of two or three feet from 

 the window of a darkened room, in which was a hole 

 three-fourths of an inch broad, to admit the light, he 

 placed a black meet of pafteboard, having in the middle 

 a hole about a quarter of an inch fquare, and behind 

 the hole the blade of a (harp knife, to intercept a fmall 

 part of the light which would otherwife have pafied 

 through the hole. The planes of the pafteboard and 

 blade were parallel to each other, and when the pafte- 

 board was removed at liich a diftance from the win- 

 dow, as that all the light coming into the room muft 

 pafs through the hole in the pafteboard, he received 

 what came through this hole on a piece of paper two 

 or three feet beyond the knife, and perceived two 

 ftreams of faint light fhooting out both ways from the 

 beam of light into the fhadow. As the brightnefs of 

 the direct rays obfcured the fainter light, by making, 

 a hole in his paper he let them pafs through, and had 

 thus an opportunity of attending clofely to the two 

 ftreams, which were nearly equal in length, breadth, 

 and quantity of light. That part which was neareft to 

 the fun's direct light was pretty ftrong for the fpace of 

 about a quarter of an inch, decreafing gradually till it 

 became imperceptible, and at the edge of the knife it 

 fubtended an angle of about twelve or at moft four- 

 teen degrees. 



Another 



