OF NEWf ON'S OPTICS. 



61 



in it, the diameter of which was the 

 42nd part of an inch. Through this hole 

 he admitted a beam of the sun's light, 

 and found that the shadows of hairs, 

 pins, straws, and such slender sub- 

 stances, appeared, when received upon 

 a screen, to be much larger than 

 they would be if the light passed their 

 edges in unbroken straight lines. A 

 human hair, the breadth of which was 

 the 230th of an inch, held at 12 feet from 

 the hole, cast a shadow upon a screen 

 held at 4 inches from it, which was the 



T. 



GOth part of an inch broad, that is, above 

 four times broader than the hair. Like 

 effects were observable with other opaque 

 bodies, and at all distances. 



These effects were found to be entirely 



independent of the refractive power of 



the air, or of the nature of the body whose 



shadow was formed. When the hair was 



surrounded with moisture, or inclosed 



with a liquid between glass planes, the 



effect was the same. The scratches 



made on glass, and the veins in plates 



of that material, produced shadows of 



the same kind. Thus whatever was the 



origin of this effect, it was evident, that 



it did not proceed from the refractive 



powers of bodies, nor on any quality 



connected with the nature or properties 



of bodies of particular species. 



The way in which Newton conceived 

 the rays to be affected in passing the 

 body may be thus explained. Let X 

 (fig. 54) be a section of the hair at right 



angles to its length. Let AD, B E, C F, 

 KN, LO, MP, be rays of light ap- 

 proaching the hair in parallel directions. 

 The ray A D is deflected at D in the di- 

 rection D G, and falls upon the screen at 

 G. In like manner the ray K N, at the 

 same distance below the hair, is deflected 

 in the direction N Q, meeting the screen 

 at Q. Newton supposes that the force 

 which deflects the light diminishes as the 

 distance from the hair increases. Con- 

 sequently, the rays B E and L O will be 

 less deflected than AD and K N, and 

 their deflected directions E H and O R 

 will cross those of the nearer rays, if the 

 screen be sufficiently distant from the hair. 

 In like manner, the more distant rays 

 C F and M P will be still less deflected, 

 and will also cross both the last rays. 

 This continues until the distance of the 

 ray from the hair becomes so great, that 

 the deflecting power is lost, and the rays 

 proceed in straight lines to the paper as 

 T I and V S. "These rays at I and S 

 bound the shadow, and at the hair in- 

 clude within them the deflected rays, all 



of which they must cross between the 

 hair and the screen. 



The continued intersections of the de- 

 flected rays with each other between the 

 hair and the screen form curves, which 

 are concave towards the shadow, and the 

 distance between which increases as 

 the distance from the hair increases. It 

 is evident also that the illuminated space 

 on the screen, immediately beyond the 

 boundaries L S of the shadow is exposed 

 to the light of the deflected rays between 

 T I or V S and the hair, as well as to the 

 direct rays which proceed without T I 

 and V S and parallel to them. 



The shadows of bodies, such as 

 metals, stones, glass, wood, horn, ice, 

 &c., exposed to light in this way, were 

 skirted with three coloured fringes. The 

 colours observed by Newton in these 

 fringes were as follows: the first or 

 innermost fringe was violet, deep blue, 

 light blue, green, yellow, and red. The 

 second and third fringes which imme- 

 diately succeeded this were blue, yel- 

 low, red ; but their colours were faint. 



