144 COLOURS OF THIN PLATES. 



beyond the lens. The light reflected from this portion, 

 according to the Newtonian theory, should not surpass that 

 of the dark rings in intensity. The roughest trial is suffi- 

 cient to show that the intensities of the light in the two 

 cases are widely different, and thus to prove that the dark 

 rings cannot arise, as they are supposed to do in the theory 

 of the fits, from the suppression of the second reflexion. 



(157) When a pencil of light falls upon two plates in suc- 

 cession, some of the many portions into which it is divided 

 by partial reflexion at the bounding surfaces, are frequently 

 in a condition to interfere, and to give rise to the phenomena 

 of colour. 



Thus, when light is transmitted through two parallel 

 plates, slightly differing in thickness, the colour is the same 

 as that produced by transmission through a single plate, 

 whose thickness is the difference of their thicknesses, and is 

 found to be independent of the interval of the plates. This 

 phenomenon was observed by Nicholson ; and it has been 

 shown by Young to arise from the interference of two pencils, 

 one of which is twice reflected within the first plate, and the 

 other twice reflected in the second, It is obvious, in fact, 

 that if t be the thickness of the first plate, and t' that of the 

 second, the first pencil will have traversed the thickness 

 3^ + t' in glass, and the second the thickness 3^ -f t, the 

 spaces traversed in air being the same ; so that the interval 

 of retardation is the time of describing the space 2(t- t') in 

 glass. Sir David Brewster observed a similar case of inter- 

 ference, produced by two plates of equal thickness slightly 

 inclined; the thickness traversed in the two plates being 

 altered by their inclination. 



(158) In the foregoing cases, the interfering pencils are 

 mixed up with, and overpowered by, the light directly trans- 



