COLOURS OF THIN PLATES. 131 



I. In homogeneous light, the rings are alternately bright 

 and black ; the thicknesses corresponding to the bright rings 

 of succeeding orders being as the odd numbers of the natural 

 series, and those corresponding to the black rings as the in- 

 termediate even numbers. 



II. The thickness corresponding to the ring of any given 

 order varies with the colour of the light, being greatest in 

 red light, least in violet, and of intermediate magnitude in 

 light of intermediate refrangibility. In white or compound 

 light, therefore, each ring will be composed of rings of 

 different colours, succeeding one another in the order of their 

 refrangibility. 



III. Tha thickness corresponding to any given ring varies 

 with the obliquity of the incident light, being very nearly 

 proportional to the secant of the angle of incidence. 



IV. The thickness varies with the substance of the re- 

 flecting plate, and in the inverse ratio of its refractive 

 index. 



(144) In order to establish the first of these laws, it is 

 necessary to employ homogeneous light. This may be ob- 

 tained by means of the prism : or we may adopt the method 

 suggested by Mr. Talbot, and illuminate the glasses with a 

 spirit lamp having a salted wick. The light of such a lamp 

 being a yellow of almost perfect homogeneity, the rings will 

 be alternately black and yelloiv ; and their number is so great 

 as to baffle any attempt to determine it. 



The law of the thicknesses corresponding to the succes- 

 sive rings is easily established. Let be the point of con- 

 tact of the plane and spherical surfaces, and aa', W, cc', &o. 

 the diameters of the successive 

 rings formed round that point c/& a ale 



as a centre. It is evident that /^P' * /3 



the thicknesses of the plate of 



K2 



