58 



A POPULAR ACCOUNT 



But if the colour was varied, they va- 

 ried their magnitude. In the red they 

 were largest ; in the indigo and violet 

 least ; and in the intermediate colours, 

 yellow, green, and blue, they were of 

 several intermediate sizes, answering to 

 that colour. 



Hence it was plain, as in the case of 

 the lenses, that when the speculum was 

 illuminated with white light, the several 

 colours of the rings were produced by 

 the superposition of the rings which 

 were separately formed by projecting 

 successively on the speculum the several 

 component elements of white light. The 

 several tints of the rings produced by 

 the white light admitted, in this case, of 

 being found by such a scale as was ex- 

 plained in 61. 



The diameters of the corresponding 

 rings of different colours varied with the 

 refrangibility of the light, and varied 

 exactly according to the same propor- 

 tion, as in the case of the rings seen 

 between the lenses. 



In order to ascertain whether the co- 

 lours seen upon the screen were the 

 mere effects of light and shade, and not 

 to be attributed to the lights proceeding 

 from the mirror, Newton placed his eye 

 where the rings appeared plainest, and 

 directing his view towards the mirror, he 

 beheld the speculum all tinged over with 

 waves of colours, like those seen be- 

 tween the lenses ; and, like these, the 

 rings swelled and contracted as they 

 were viewed more or less obliquely by 

 moving the eye from or towards the 

 centre of the speculum. A bystander, 

 during this experiment, observed upon 

 the eye of the observer the same co- 

 loured light as he perceived in the spe- 

 culum. 



On comparing the manner in which 

 these rings were produced with that in 

 which the like phenomena were caused, 

 as described in Chapter VI., the only 

 differences which are observable are the 

 thickness of the glass, and the posterior 

 surface of it being silvered. The thin 

 transparent medium, forming a soap- 

 bubble, has the same figure as the spe- 

 culum used in the present case, with 

 this difference, that the colours are 

 seen on the convex side, whereas, in the 

 present, they are seen in the concave 

 side. This circumstance, however, so 

 far from impairing the analogy, renders 

 it more perfect when the order of co- 

 lours is considered, for the rings are not 

 those seen by reflexion in thin trans- 

 parent ^mediums, but by transmission. 

 Hence, if the soap-bubble were viewed 



upon the concave side by an eye placed 

 within it, the colours would be identical 

 with those exhibited by the concave spe- 

 culum in the present case. 



To ascertain whether the silvering 

 upon the back of the speculum had any 

 part in producing the phenomena of the 

 rings, Newton tried the effect of a simi- 

 lar concave glass, without silvering. 

 The result was the production of the 

 same rings, but with more faint colours, 

 owing to the reflecting power of the 

 second surface being diminished by the 

 want of silvering. It therefore appeared 

 that the circumstance of the back of the 

 speculum being silvered, had no other 

 effect than that of increasing the inten- 

 sity of the colours. 



Notwithstanding the identity of the 

 two phenomena, as well as the complete 

 similitude which existed between the 

 manner in which they were produced, 

 Newton did not feel himself warranted 

 in applying the theory of the fits of 

 easy reflection and transmission in ac- 

 counting for them, until he established 

 the fact that the two surfaces of the glass 

 were indispensable for their production. 

 Were it possible for a single surface, i. e. 

 the concave surface alone, to produce 

 the phenomena, this theory would have 

 been quite inapplicable, and altogether 

 inadequate to explain them. To reduce 

 this question to the test of experiment, 

 Newton procured a concave speculum 

 of polished metal, which reflected only 

 from one surface. On presenting this 

 to the light, in the same manner as the 

 glass speculum, no rings were produced. 

 This result was decisive of the point, and 

 proved that the two surfaces were neces- 

 sary to the production of the rings. 



It was plain that the light, regularly 

 reflected from the speculum, had no part 

 in these phenomena; for, as all the in- 

 cident light radiated from the centre of 

 the sphere, it fell perpendicular on the 

 speculum, and was therefore reflected 

 back perpendicularly to the same centre. 

 The rings, therefore, must owe their ex- 

 istence to the light irregularly scattered 

 by the surfaces of the glass. By the 

 experiment already mentioned to have 

 been made with the metallic speculum, 

 it appeared that they could not be pro- 

 duced by the light scattered by the irre- 

 gular reflection of the first surface alone ; 

 for, if that were possible, the metallic 

 speculum, by the surface of which the 

 light was thus scattered, would have ex- 

 hibited them. The cause of the pheno- 

 mena was, therefore, to be looked for in 

 the light irregularly refracted by the 



