192 NEWTON'S RINGS. [SECT. xx. 



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 betVeen 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 surfaces 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 ex- 

 plained 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 confirmed 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 con- 

 tact reaches the eye by paths of different lengths, and pro- 

 duces 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. 



The size of the rings increases with the obliquity of the in- 

 cident light, the same colour requiring a greater thickness 

 or space between the glasses to produce it than when the light 

 falls perpendicularly upon them. Now, if the apparatus be 

 placed in homogeneous instead of white light, the rings will 

 all be of the same colour with that of the light employed ; 

 that is to say, if the light be red, the rings will be red, divided 



