42 



A POPULAR ACCOUNT 



eluded, that these effects being severally 

 known, there could be no difficulty in 

 combining them, so as to account for the 

 phenomena produced by compound solar 

 light. 



With this view, he decomposed a sun- 

 beam, by means of a prism, and casting 

 successively on the lenses the several co- 

 loured lights in the spectrum, he ob- 

 served and carefully noted the pheno- 

 mena. In each case the rings appeared, 

 and even in greater numbers, than in the 

 case of the compound solar light. They, 

 however, no longer exhibited any variety 

 of colour, the central spot being now 

 surrounded by rings of the same colour 

 as the light cast upon the lenses, sepa- 

 rated by dark rings, in which, like the 

 central spot, all light seemed to be trans- 

 mitted, and none reflected. Upon look- 

 ing through the lenses towards the light, 

 the intermediate rings just mentioned, as 

 well as the central spot, appeared of the 

 colour of the prismatic light, to which 

 the lenses were exposed ; and, on the 

 other hand, those rings which by reflect- 

 ed light appeared coloured, were now 

 dark, no light being transmitted. 



Let S S, fig. 43, be a section of the 

 air-lens, and suppose a beam of homo- 



Fig. 43. 



geneous red light projected on it from 

 the direction A, and perpendicular to its 

 surface. The centre of the lens being o, 

 let 1 be the place of the first ring of red 

 light, as viewed from A. At 2 will be a 

 dark ring, at 3 a second ring of red light, 

 at 4 a dark ring, and so on, the central 

 spot o being dark. Now, let the lens be 

 viewed from B, so as to receive the rays 

 transmitted through it. The central spot 

 o will appear red, the first ring 1, which 

 before was red, will be dark, the ring 2 

 will be red, 3 dark, and so on ; all the 

 rings which were dark, when viewed 

 from A, being red when viewed from B, 

 and vice versa. 



Upon exposing the lenses to orange, 

 yellow, and the other species of homoge- 

 neous light, similar effects were observ- 

 able ; the bright rings always taking the 

 colour of the light incident on the lenses, 

 and being separated by dark rings, which 

 being viewed from B appeared bright, 



the bright ones, as in the case of the red 

 light, appearing dark. One difference, 

 however, was remarkable, viz. that the 

 rings formed by the less refrangible rays 

 were larger than those formed by the 

 more refrangible. Thus the first red 

 ring was larger than the first orange one 

 and this larger than the first yellow ring, 

 and so on, the first violet ring being least. 



(55.) The existence of a connexion be- 

 tween the colour of the reflected and 

 transmitted light, and the thickness of 

 the air-lens, being now manifest, Newton 

 applied his attention to measure this 

 thickness at the places of the several 

 dark and bright rings. The extreme 

 minuteness of the magnitude to be as- 

 certained rendered the application of 

 direct measures impracticable. The first 

 principles of elementary geometry, how- 

 ever, furnished a method of effecting the 

 measurement with the greatest accuracy. 



* Let O T,/g-. 44, be the plane side of 



Fig. 44. 



the air-lens, and C D the concave side, 

 and let the circle, of which C D is an 

 arc, be completed. Let O B be its dia- 

 meter. By the principles of optics, the 

 length of O B may be deduced, from ob- 

 serving the focus of the convex glass 

 lens used in the experiments, provided the 

 refracting power of the glass be known. 

 Let it be required to ascertain the thick- 

 ness T P of the air-lens at T. Draw the 

 line P O, and from P draw PA parallel 

 to T O. By actually measuring the dia- 

 meter of the ring at the distance T, the 

 line T O, or P A, will be determined ; and 

 from the extreme minuteness of T P, the 

 line PA may be considered as practically 

 equal to P O. The right angled triangles 

 OPB, and OAP, are similar; and, 



* This investigation may be omitted by those who 

 are not familiar with the eiements of geometry. 



