146 



COLOURS OF THIN PLATES. 



mirror, having a hole in the middle to let the beam pass and 

 repass, a set of broad coloured rings will be depicted on it, 

 similar to the transmitted rings of thin plates. The diameters 

 of these rings vary inversely as the square roots of the thick- 

 nesses of the mirrors. 



When the mirror is inclined a little, so as to throw the 

 reflected image a little to one side of the aperture, the rings 

 are formed as before ; but their centre is in the middle of the 

 line joining the aperture and its image. At this centre is a 

 spot, which changes its appearance in a remarkable manner 

 as the image recedes from the aperture, being alternately 

 dark and bright in homogeneous light, and in white light 

 assuming every gradation of tint in rapid succession. 



(160) These phenomena have been shown to arise from 

 the interference of the two portions of light, which are irre- 

 gularly scattered in the passing and repassing of the ray 

 through the refracting surface. 



Thus, let be the aperture through which the beam is ad- 

 mitted, and allowed to 

 fall perpendicularly on 

 a reflecting plate at A 

 and B. A portion of 

 the incident light will 

 be irregularly scat- 

 tered, in the passage of 

 the ray OAB through 



'the first surface of the plate ; and this portion will diverge 

 from the point A in all directions. Let AC be one of the 

 rays which compose this scattered portion : this is reflected 

 at the second surface of the plate in the direction CD, and 

 emerges in the direction DM. But the direct ray, AB, 

 which is reflected back in BA, will again be partially scat- 

 tered in repassing through the first surface. Let AM be one 



