COLOURS OF THIN PLATES. 145 



mitted, and some contrivance is necessary to make the colours 

 visible. The phenomena are^ much more obvious in the light re- 

 flected from both plates, which, on account of their inclination, 

 is thus separated from the direct light. It is obvious, in fact", 

 that the direct image of a luminous object, seen through two 

 glasses slightly inclined, will be accompanied by several 

 lateral images, formed by 2, 4, 6, &c. reflexions. These 

 images Sir David Brewster observed to be richly coloured ; 

 the bands of colour being parallel to the line of junction of 

 the two glasses, and their breadth being greater, the less the 

 inclination of the plates. The colours in the first lateral 

 image are produced by the interference of the two pencils 

 ABCDEFGH, ABCdefgh, into 



which the ray is divided at the / 



first surface of the second plate ; 



one of these portions being re- v/\ 



fleeted externally by the second 

 plate, and internally by the first, 

 while the other is reflected in- 

 ternally by the second, and exter- 

 nally by the first. The routes of 

 these portions are obviously equal 



when the plates are parallel, and differ in length only by 

 reason of their inclination. 



(159) The two preceding cases of interference may be 

 produced with plates of any thickness. What are termed the 

 colours of thick plates, however, are phenomena of another 

 kind, and arise in circumstances wholly different. These 

 phenomena were first observed by Newton. 



In Newton's experiment a beam of light is admitted 

 through a small aperture, and received on a concavo-convex 

 mirror with parallel surfaces, the hinder of which is silvered. 

 A screen of white paper being then held at the centre of the 



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