OPTICAL XÖTE. 



195 



dark riii,U" tlie light of maximum iiiteusity is made minimum by mica 

 and in the white rings, the straight liands are not in the middle pai-t 

 between any two dark ciir\ ed bands. Five groups were well obser\ed, 

 and oToiips of a hiii'her order mio-ht be observed 1)v means of tliinner 

 mica. It is needless to say that air tihns corresponding to the central 

 dark Ijands A, !>, C, dkc. are m the ratio of 1, 2, 3, Arc, as is jiroved 

 bv the fact that the s(|nares of the diameters of the dark rings nre in 

 tlie same rati<). 



The difference of march between once and twice reflected rays 

 bv the air film corresponding to the middle dark band of A together 

 with the loss of i. / is precisely eqnal to that introduced Ijy the mica 

 fihn, as is shown by the fact that the dark band produced by the air 

 film lies mid- way Ijetween the two bands produced l)y the mica. 

 Hence the thinner the film, the nearer is the group A to the centre of 

 the system. 



In tlie faint rings, the difference of march intrtxluced by the air 

 film corresponding to the dark middle ring is greater than that by 

 the mica film Ijy y /- Hence if we could observe its spectrum, there 

 would be twice as manv straio-lit bands as curved ones, but I was not 

 able to observe it on account of faintness. 



The effect of inclining the mica fihn so as to recei\'e the rays 

 more oblicjuely is e(pii\ aient to thinning the film as regards the 

 difference of march introduced. Hence if the grou[) A is not seen 

 within the field on account of too ü'reat a thickness of the mica film, 

 it can be seen if the film is held obli(piely. 



The groups are als() well observed l)y refiected liglit from the 

 front surface of the mica film. It is needless to say that the 

 appearances are complementary. 



