LXIV. Proceedings of Learned Societies, 



CAMBRIDGE PHILOSOPHICAL SOCIETY. 

 ' [Continued from vol. i. p. 568.] 

 May 19,|r^N the Colours of Thick Plates. By G. G. Stokes, M.A., 

 1 85 1 . ^-^ Fellow of Pembroke College, and Lucasian Professor 

 of Mathematics in the University of Cambridge. 



By the expression " colours of thick plates " is usually understood 

 the system of coloured rings, discovered by Newton, which are formed 

 on a screen when the sun's light is transmitted through a small hole 

 in the screen, and received perpendicularly upon a concave mirror of 

 quicksilvered glass, placed at such a distance from the screen that 

 the image of the hole is at the same distance from the mirror as the 

 hole itself. The brilliancy of the rings, as was afterwards discovered, 

 is greatly increased by tarnishing the surface of the mirror ; and it 

 is also advantageous to use a lens to collect the sun's rays, and to 

 place the screen so that the small hole may be situated at the focus 

 of the lens. These rings were first explained on the undulatory 

 theory by Dr. Young, who attributed them to the interference of 

 two streams of light ; of which the first is scattered at the tarnished 

 surface of the mirror, and then regularly reflected and refracted, while 

 the second is regularly refracted and reflected, and then scattered in 

 coming out of the glass. The theory has been worked out in detail 

 by Sir John Herschel, who has investigated the case in which the 

 two surfaces of the glass belong to a pair of concentric spheres, and 

 the hole in the screen is situated in the common centre of curvature. 



A set of coloured bands has since been observed by Dr. Whewell 

 in a common plane mirror. These bands are seen when a candle is 

 held near the eye, at the distance of several feet from the mirror, 

 and is viewed by reflexion. It is necessary that the first surface of 

 the glass should be a little tarnished. The theory of these bands 

 had not been worked out, and it had even been doubted by some 

 philosophers whether they were of the nature of the coloured rings 

 of thick plates. 



In this paper the author gave a general investigation, which in- 

 cludes as particular cases the theory of the rings formed on a screen 

 in Newton's experiment, and that of the bands which Dr. Whewell 

 had observed in a plane mirror, and which are not thrown on a 

 screen, but viewed directly by the eye. He also exhibited to the 

 meeting a variation of Newton's experiment, in which an extremely 

 beautiful system of rings is very easily produced without sunlight. 

 The face of a concave mirror of quicksilvered glass was prepared by 

 pouring on it a mixture consisting of one part of milk to three or 

 four of water, and then holding the mirror vertically in front of a 

 fire to dry. When the flame of a taper, or of an oil-lamp with a 

 small wick, is placed in front of a mirror thus prepared, in such a 

 position as to coincide with its inverted image, a beautful system of 

 rings is seen encompassing the flame. These rings appear to have 

 a definite position in space, like a bodily object. The rings thus 



