SECT. xx. NEWTON'S RINGS: 165 



Now, as the velocity of light is known to be 190,000 

 miles in a second, if the length of the waves of the dif- 

 ferent colored rays could be measured, the number of 

 vibrations in a second corresponding to each could be 

 computed ; that has been accomplished as follows : 

 All transparent substances of a certain thickness, with 

 parallel surfaces, reflect and transmit white light ; but 

 if they be extremely thin, both the reflected and trans- 

 mitted light is colored. The vivid hues on soap-bubbles, 

 the iridescent colors produced by heat on polished steel 

 and copper, the fringes of color betweefa the laminae of 

 Iceland spar and sulphate of lime, all consist of a suc- 

 cession of hues disposed in the same order, totally inde- 

 pendent of the color of the substance, and determined 

 solely by its greater or less thickness, a circumstance 

 which affords the means of ascertaining the length of 

 the waves of each colored 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 curva- 

 ture, before an open window; when they are pressed to- 

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

 surrounded by seven rings of vivid colors, all differing 

 from one another (N. 194). In the first ring, estimated 

 from the black spot, the colors 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 colors are 

 pale bluish-green and very pale pink. That these rings 

 are formed between the two surfaces in apparent con- 

 tact may be proved by laying a prism on the lens, in- 

 stead of the plate of glass, and viewing the rings through 

 the inclined side of it that is next to the eye, which ar- 

 rangement prevents the light reflected from the upper 

 surface mixing with that from the surfaces in contact, so 

 that the intervals between the rings appear perfectly 

 black, one of the strongest circumstances in favor 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 undu- 

 latory theory, the intervals between the rings ought to 

 be absolutely black, which is confirmed by experiment ; 

 whereas by the doctrine of emanation they ought to be 



