[ 363 ] 



XLII1. On certain coloured Bands observed in examining New- 

 ton's Mings. By A. R. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



I N examining Newton's rings I was induced to place a con- 

 vex lens (36-inch focus) between two surfaces of plate- 

 glass, in order to effect the superposition of the rings. 



My attention was suddenly diverted when viewing the rings 

 with transmitted light, by observing a series of coloured 

 bands extending nearly over the whole surface of the lens, 

 but not assuming any definite form, nor did the breadth or 

 direction of any of these bands bear the slightest relation to 

 the situation of the rings; their curvature varied materially 

 with a variation in the pressure ; and their breadth changed 

 from the same cause. Having satisfied myself that these ap- 

 pearances were not owing to any films, or particles of dust, I 

 endeavoured to obtain them by reflection, and succeeded 

 equally well. In examining them with glass of different thick- 

 ness, the magnitude and curvature of the bands differed ma- 

 terially for the same degree of pressure, and it appeared that 

 for extent of surface and brilliancy of colour, two pieces of 

 plate glass of equal thickness are the best. These effects were 

 observed by the light of a candle, and on the following day 

 I found that they were not produced by the sunlight until re- 

 ceived into a darkened room through a very small orifice, when 

 the maximum effect was produced by placing the lens at its 

 focal distance from the orifice. The breadth of the bands is 

 greatest for the same pressure when the incident beam of light 

 is Z. x to the surface of the glass. 



Polarized light produced no change, except when the ana- 

 lysing eye-piece of Iceland spar was in such a position as not 

 to transmit the reflected beam, in which case the darkness was 

 too intense to render the colours visible. I have repeated this 

 experiment with glass varying' in thickness from the twelfth of 

 an inch to three fourths of an inch; in the latter the bands 

 could scarcely be discovered, and were then visible only in 

 the immediate neighbourhood of the rings. 



These are the principal facts I have observed, and their 

 explanation will probably be found to depend upon the inter- 

 ference of the waves of light reflected and transmitted at the 

 different surfaces of the glass, and this is partly confirmed 

 from my being unable to obtain the bands when a lens of short 

 focal length is used. Whether this experiment will aid in 

 establishing the greater certainty of any branch of the undu- 



3 A2 



