OF NEWTON'S OPTICS. 



57 



medium. A part is reflected, according 

 to the regular law of reflection, and a part 

 entering the medium is regularly re- 

 fracted. But besides these, which may be 

 considered as the principal parts, there is 

 still another portion which is scattered 

 about in all directions around each point 

 of incidence, both within and without 

 the refracting medium. It is this portion 

 of the incident light which serves to 

 render the surface visible to an eye 

 placed in any situation with respect to 

 it. For if no light proceeded in any 

 other directions than those of the regu- 

 larly reflected and refracted rays, the 

 surface would be invisible to every eye, 

 except to one placed in these directions. 

 It is to the portion of light scattered 

 thus irregularly that the phenomena, 

 which we are now about to describe, 

 are to be traced. 



Newton procured a concave glass 

 speculum or mirror, silvered on the con- 

 vex side, and in every part exactly a 

 quarter of an inch thick. This he placed 

 in a darkened chamber opposite the 

 window, through which a beam of light 

 was admitted. The radius of the sphere on 

 which the mirror was ground was about 

 six feet, so that the centre of the mirror 

 was at that distance from its surface. 

 At the centre of the mirror he placed a 

 white opaque paper screen, having a 

 small hole in it, and so adjusted the 

 mirror and the screen, that the beam of 

 light, passing through the hole in the 

 screen, should fall upon the mirror, and 

 be reflected back from the mirror to the 

 same hole. 



Let AB (fig. 50) be the mirror, C D 



Fig. 50. 



Ihe screen placed before it, so that the 

 hole O shall be at the centre of the mirror. 

 The rays of light, passing from O to the 

 mirror, will fall perpendicularly on it, 

 and consequently all the regularly re- 

 flected rays will return to O ; and as far 

 as these rays are concerned, the side of 

 the screen presented to A B will be as 

 dark as if no light passed through to 



the mirror. Nevertheless, upon observ- 

 ing the screen presented towards A B, 

 Newton found this not to be the case. 

 He observed upon the paper " four or 

 five concentric irises or rings of colours 

 like rainbows, encompassing the hole. 

 These rings, as they grew larger, became 

 fainter and diluter, so that the fifth was 

 scarce visible. Yet, sometimes, when 

 the sun shone very clear, there appeared 

 to be faint lineaments of a sixth and a 

 seventh." 



There was a very obvious analogy be- 

 tween these rings, and those exhibited 

 between the lenses in 54. The co- 

 lours did not succeed each other in 

 the order of the reflected rings, but in 

 that of those which were in that case 

 transmitted. In the centre, at and 

 around O, was a white round spot. 

 This was skirted or fringed with a dark 

 grey, which insensibly brightened into a 

 violet ring. This was followed by a 

 circle of indigo, one of pale blue, a 

 greenish yellow, a vivid pure yellow, and, 

 finally, a red, which deepened into a 

 purple on Ihe outer edge. Such was the 

 first iris which surrounded the white 

 central spot. 



This was encompassed by a second 

 series of coloured rings, of which the 

 first was a dark purple, the outer boun- 

 dary of the red in the former series. 

 This was followed in succession by cir- 

 cles of blue, green, yellow, and red. 



The colours of the third and fourth 

 series were green and red. Those of the 

 fifth were so faint as to be scarcely dis- 

 tinguishable. 



In order to trace their connexion with 

 rings exhibited by air inclosed between 

 the lenses, Newton now measured their 

 diameters, and found exactly the same 

 proportion subsist among them as pre- 

 vailed among the rings seen by trans- 

 mission in that case. In order to make 

 the relation of these phenomena still 

 more manifest, he now transmitted 

 through the hole O, not a beam of com- 

 pound solar light, but of pure homoge- 

 neous light, obtained in the usual way 

 by a prism. Transmitting through O in 

 succession each of the colours, he ob- 

 served the effects on the screen. Rings 

 now only appeared of that colour which 

 fell upon the speculum. If the specu- 

 lum were illuminated with red, the rings 

 were totally red with dark intervals ; if 

 with blue, they were totally blue, and so 

 of the other colours. With whatever 

 colour they were illuminated, the same 

 proportion subsisted among their dia- 

 meters as was observed with the lenses. 



