COLOURS OF THICK PLATES. [169] 



Section V. 



On the nature of the deflection of the two interfering streams from the course of the 

 regularly refracted light. 



27. It was suggested to me by a friend, to whom I was shewing some of the experiments 

 described in this paper, that in order to see bands or rings of the same nature it would 

 probably be sufficient to dim the two faces of a plate of glass, and view a luminous point 

 through it. But having prepared the two faces of a piece of plate glass with milk and water 

 in the same manner as for mirrors, taking care to treat the two faces as nearly as possible alike, 

 on viewing a luminous point through the plate I found not the slightest trace of the rings or 

 bands, whatever were the distance of the eye from the plate. There were indeed one or two 

 indistinct rings surrounding the luminous point ; but these were of a totally different nature, 

 being analogous to the rings seen with lycopodium seed, and arising from the interference of 

 pairs of streams which passed on opposite sides of the milk globules. There was no difficulty 

 in distinguishing them from the system sought for, since they continued to have the luminous 

 point for their centre when the plate was inclined to the line joining that point with the 

 eye. The absence of rings or bands indicates therefore that the streams scattered at the 

 opposite sides of a plate are incapable of interfering. 



The rays scattered so as to make infinitely small angles with the regularly refracted rays 

 belong to a point in the bright band of the order zero, and are therefore brought to a focus 

 on the retina when the luminous point is seen distinctly. The same must be at least very 

 approximately true for neighbouring points of the system of rings, did any such exist, and 

 therefore a portion at least of the system would be seen distinctly when the luminous point was 

 seen distinctly. The distances of the luminous point from the glass plate, and of the glass 

 plate from the eye, were comparable with the corresponding distances in the experiment with 

 a plane mirror, and the thickness of glass was comparable likewise; and with a mirror the 

 bands are seen with the utmost facility within wide limits of the thickness of the glass, and of 

 the distances of the luminous point and of the eye from the mirror. But to prevent any 

 doubt as to whether the bands might not have been too small to be seen when formed by trans- 

 mission, I have calculated the retardation in the same manner as in Art. 10. The result is 



2 



M 



where R is the retardation of the stream scattered at emergence relatively to that scattered 

 at entrance, c is the distance of the luminous point from the plate, h that of the plate from 

 the eye, t the thickness, and /x the refractive index of the plate, and x, y are the co-ordinates 

 of the point in which the plate is cut by any small pencil (regarded as a ray) which enters the 

 eye, and are measured from the point in which the plate is cut by a line joining the luminous 

 point and the eye, a line to which the plane of the plate is supposed to be perpendicular. 

 On substituting numerical values in the above formulas, it appeared that the dimensions of the 

 rings were such that they could not possibly have escaped notice had they really been formed. 

 The non-appearance of the rings leads to the following law. 



Vol. IX. Part II. 46 



