SECT. XX. NEWTON'S RINGS. . 171 



Iceland spar and sulphate of lime, all consist of a succession of 

 hues disposed in the same order, totally independent of the 

 colour of the substance, and determined solely by its greater or 

 less thickness a circumstance which affords the means of ascer- 

 taining the length of the waves of each coloured ray, and the 

 frequency of the vibrations of the particles producing them. If 

 a plate of glass be laid upon a lens of almost imperceptible 

 curvature, before an open window, when they are pressed toge- 

 ther a black spot will be seen in the point of contact, surrounded 

 by seven rings of vivid colours, all differing from one another 

 (N. 199). In the first ring, estimated from the black spot, the 

 colours succeed each other in the following order : black, very 

 faint blue, brilliant white, yellow, orange, and red. They are 

 quite different in the other rings, and in the seventh the only 

 colours are pale bluish green and very pale pink. That these 

 rings are formed between the two surfaces in apparent contact 

 may be proved by laying a prism on the lens instead of the 

 plate of glass, and viewing the rings through the inclined side of 

 it that is next to the eye, which arrangement prevents the light 

 reflected from the upper surface mixing with that from the sur- 

 faces in contact, so that the intervals between the rings appear 

 perfectly black one of the strongest circumstances in favour of 

 the undulatory theory ; for, although the phenomena of the 

 rings can be explained by either hypothesis, there is this material 

 difference, that, according to the undulatory theory, the intervals 

 between the rings ought to be absolutely black, which is con- 

 firmed by experiment ; whereas, by the doctrine of emanation, 

 they ought to be half illuminated, which is not found to be the 

 case. M. Fresnel, whose opinion is of the first authority, thought 

 this test conclusive. It may therefore be concluded that the 

 rings arise entirely from the interference of the rays : the light 

 reflected from each of the surfaces in apparent contact reaches 

 the eye by paths of different lengths, and produces coloured and 

 dark rings alternately, according as the reflected waves coincide 

 or destroy one another. The breadths of the rings are unequal ; 

 they decrease in width, and the colours become more crowded, 

 as they recede from the centre. Coloured rings are also produced 

 by transmitting light through the same apparatus; but the 

 colours are less vivid, and are complementary to those reflected, 

 consequently the central spot is white. 



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