142 COLOURS OF THIN PLATES. 



former and the other by the latter. These two portions will 

 interfere, and produce fringes after reflexion ; and it is found 

 that in the two former cases the central band is white, the 

 two waves being in complete accordance : in the third case 

 i. e. when one of the pencils is reflected from the rarer, and 

 the other from the denser medium the central band is a 

 black one ; the two waves are, therefore, in complete discord- 

 ance, and their phases differ by half an undulation. 



It follows from the preceding, that in the system of rings 

 formed between two object-glasses, the central spot will be 

 white, if the thin plate is of a density intermediate to those of 

 the two glasses ; for it is evident that the reflexion takes 

 place under the same conditions at the two surfaces i. e. in 

 both cases at the surface of a rarer, or in both at that of a 

 denser medium. Young verified this anticipation by inclos- 

 ing oil of sassafras between two object-glasses, one of which 

 was of flint-glass, and the other of crown-glass. 



(155) We have spoken of another set of rings visible by 

 transmission. These are produced by the interference of the 

 rays directly transmitted through the plate with those which 

 penetrate it after two interior reflexions. It follows from the 

 preceding considerations that they should be complementary 

 to those seen by reflexion ; and this is observed to be the 

 case. The extreme paleness of the transmitted rings arises 

 from the great difference in the intensities of the interfering 

 pencils. 



(156) The theory of thin plates, as it came from the 

 hands of Young, laboured under an imperfection, which was 

 however soon removed. It is obvious that the intensities of 

 the two portions of light, reflected from the upper and under 

 surfaces of the plate, can never be the same, the light inci- 

 dent on the second surface being already weakened by partial 



