302 ON LIGHT. 



diminution) will be returned, and, passing back through 

 the first surface (with only the same trifling per-centage of 

 loss), will, on emerging, be superposed on the first reflected 

 series, with which it will be coincident in direction. If 

 then the thickness of the plate be such that in passing 

 to the hinder surface, back again, and out at the first, the 

 second series shall have lost upon the first precisely one, 

 three, five, or any odd number of ^^/-undulations, it 

 will begin to emerge in the exact opposite " phase " of 

 its period to that of the undulation of the first reflected 

 series which starts from the same point at the same in- 

 stant. Here, then, we have the case contemplated above 

 of two series of equal waves entering the same " chan- 

 nel" in opposite phases. They will therefore destroy 

 each other, or the intensity of the joint ray will be nil. 

 The contrary will happen if the thickness be such as to 

 produce a retardation of two, four, or any even number 

 of semi-undulations. In that case the two reflected rays 

 will conspire and produce a joint one of double inten- 

 sity: and of intermediate in the various cases of inter- 

 mediate retardation. 



(85.) Thus we see that the' degree of brightness of the 

 reflected light depends on the thickness of the reflecting 

 film, and that for a certain series of thicknesses in arith- 

 metical progression, the joint reflection is nil; for another 

 series exactly intermediate, it attains a maximum of inten- 

 sity; and between these limits, all gradations of illumina- 

 tion will arise according to the intermediate thicknesses 

 supposed to exist. This is so far in general accordance with 

 the phenomena described : but before applying it to the 



