250 Mr. T. C. Porter on a Method 



of the rays of white light coming through the slit/, reflected 

 at the uppermost surface at m, giving the image A, then 

 transmitted through the upper plate to a and accompanied by 

 light reflected from the upper surface of the lower plate, which 

 has not interfered with it. At a a part of the ray is reflected ; 

 the rest is refracted into the air-space, undergoes reflexion 

 at b, another refraction at c; and is then transmitted to d 

 unless destroyed by interference : there, part is refracted into 

 the air and forms one of the bright primaries ; but a consi- 

 derable part is reflected to h and g, the two reflected rays there 

 interfering for the first time, and generating the continuations 

 of the primary rings ; for it is evident that light reflected from 

 d might, so far as its effect at k is concerned, have been origi- 

 nally incident at d, in which case it certainly would have 

 generated the continuations of the parts of the primary rings 

 observed in A. Thus B will show (1) a reflexion of the 

 parts of the primaries seen in A, and (2) continuations of the 

 primaries. It is now perfectly obvious that the image B 2 

 (fig. 5) ought to exhibit: — (1) Faint continuations of the 

 primary rings, produced by light which has escaped inter- 

 ference in c and h (fig. 8), but has interfered for the first 



Fiff. 8. 



LOWER PLATE 



time at s, and has therefore the same effect as if it had been 

 originally incident at /. (2) Continuations of the first re- 

 flexion of the primaries visible in B x . These continuations 

 are caused by light which has escaped interference as far aso, 

 and is the result of the internal reflexion at d, i. e. the rings 

 occupy the same position as if the light had been originally 

 incident at d. (3) A new reflexion of the primary rings, 

 first caused at c, and reflected at d, g,f, s. 



Observation shows these three sets of rings in B 2 (vide 

 figures 6 and 9, fig. 9 being added to make plain the 

 arrangement by which fig. 6 was obtained), and no more. 

 In the same way we can predict exactly what the inter- 

 ference systems will be like in Ci, C 2 , &c. We have not 



