248 Mr. T. C. Porter on a Method 



lower surface of the upper plate, (c) the external upper sur- 

 face of the lower plate, and possibly (d) the external lower 

 surface of the upper plate (vide fig. 8). 



Of (a), [b), and (c), the images A 2 , B l7 B 2 , C l5 C 2 contain 

 rays which have undergone reflexion at these respective sur- 

 faces the number of times under the corresponding letter in 

 the subjoined table 



(a). (b). (c). 



A 2 Oil 



B, 12 2 



B 2 2 3 3 



(\ 3 4 4 



C 2 4 5 5 



Now since the phase of a light-wave loses half a wave- 

 length in the act of being reflected in a denser medium, and 

 since the sum of the three rows is odd for each strip such as 

 Bj, the light which forms the figures seen in each image of the 

 slit will in every case lose an integral number of wave-lengths: 

 it follows that if the centre of the primary rings is black or 

 coloured, so far as this consideration is concerned, the centres 

 of the secondary and tertiary rings will also be black or 

 coloured. B l5 B 2 , C 1? C 2 . . . . will therefore each contain a 

 reflexion of the primary rings, growing weaker and. weaker 

 in intensity, not only from the loss of light at each re- 

 flexion but - also from absorption, and perhaps from the 

 scattering of a very small fraction of the light by solid par- 

 ticles of dust or air-bubbles, which are wont to occur even in 

 the clearest glass. The effect of the curvature of the lower 

 surface of the upper plate will be to displace these repetitions 

 of the primary rings a little downwards. It is clear that any 

 rays which once completely interfere are cut out once for all ; 

 and it follows that any interference -curves which appear for 

 the first time in any particular image of the slit must be the 

 result of light which has hitherto escaped interference : e. g. 

 if we look at the image B„ besides the reflexion of the primaries 

 there is a series of rings which are exact continuations of the 

 primaries ; and, moreover, these continuations can be traced, 

 though the observation is not an easy one, rigid across the 

 reflexion of the black central spot which occupies the centre of 

 the primary rings. If this be not due to light scattered in the 

 body of the glass itself, it must prove that the interference 

 which causes the black spot at the primaries is not complete, 

 though very nearly so. In making this observation the eye 

 must be screened from all light except that which comes 

 from the black spot in B; and it must be borne in mind that 

 care is necessary to avoid the smallest particles of dust between 



