186         ON THE COLOURS OF THICK PLATES.
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.