XIII. On the Colours of Thick Plates. By G. G. Stokes, M.A., Fellow of 

 Pembroke College, and Lucasian Professor of Mathematics in the University 

 of Cambridge. 



[Read May 19, 1851.] 



The expression " colours of thick plates, 11 has been appropriated to a class of phenomena 

 discovered by Newton, and described by him in the fourth part of the second book of his 

 Optics. In Newton's experiment, the sun's light was admitted into a darkened room through 

 a hole in the window-shutter, and allowed to fall perpendicularly upon a concave mirror, 

 formed of glass quicksilvered at the back. A white opaque card pierced with a small hole 

 being then interposed, at the distance of the centre of curvature of the mirror, so that the 

 regularly reflected light returned by the same small hole by which it entered, a set of coloured 

 rings was seen depicted on the card encompassing the hole. The existence of these rings was 

 attributed by Newton to the light scattered on entering the glass, and then regularly reflected 

 and refracted ; and he succeeded in deducing from his theory of fits the laws of the rings, 

 both as regards the relation between the diameters of successive rings, the order of the colours, 

 the variation of the diameter of a given ring corresponding to a variation either in the radius 

 of curvature of the surfaces or in the thickness of the glass, and even the absolute magnitude 

 of the system formed under given circumstances. The phenomena which present themselves 

 when the mirror is inclined a little, so as to throw the image of the hole to one side, are very 

 curious, and have been accurately described by Newton in his tenth and eleventh Observations. 



In the course of a series of experimental researches on these rings, the Duke de Chaulnes* 

 discovered accidentally that their brilliancy was greatly increased by breathing on the glass. 

 Since the moisture soon evaporated, in order to procure a permanent tarnish, he spread over 

 the surface a small quantity of a mixture of milk and water, which on drying left a degree of 

 dimness very suitable to the experiments. By substituting for the glass mirror a metallic 

 speculum, in front of which there was placed a plate of tarnished mica, it was easy to observe 

 the variation in the diameter of the rings corresponding to a variation in the distance of the 

 mica from the speculum. In this form of the experiment the glass plate was replaced by the 

 plate of air comprised between the mica and the speculum. Rings were also produced when 

 the tarnished mica was replaced by a screen of fine muslin. In this case, however, according 

 to the Duke's statement, the rings were nearly square, though rounded off" a little at the 

 angles. A set of parallel wires gave merely a bright band intersected by short bands which 

 were vividly coloured. Even the blade of a knife produced a similar appearance, weak 



* Memoires de t'Academie, 1755, p. 136. 



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