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He measured the diameters of these rings, in a particular case, and thence, knowing the curvature of the surface, he was able to calculate the thickness of the lamina at each ring.
Repeating this observation under different angles of incidence, he remarked the variations produced in the rings; he found that they grew wider as the obliquity increased, and by measuring their diameters, he calculated the different thicknesses at which the same colour appeared.
He made similar experiments on thin plates of water, contained between two glasses, and on thin soap bubbles, blown with a pipe. These bubbles being placed on a plane glass, became perfectly hemispherical, and being covered over with a bell-glass, they lasted long enough for him to observe at leisure their brilliant tints. He thus found that the thicknesses, at which the same colours appeared were less than in air, in the ratio of 3 to 4, which is, in fact, that of refraction between those two substances. Other trials with laminæ of glass, led him to generalize this remark, which many other experiments afterwards confirmed. He collected all his results into empiric tables, which express the laws of them in numbers.
These laws were, however, still complicated in consequence of the unequal refrangibilities of the different rays, by which the rings were illuminated. To reduce the phænomenon to its greatest simplicity, Newton formed rings with simple light, by looking, in a dark room, at a white paper, which received in turns all the simple colours of the prismatic spectrum. This paper thus enlightened, and seen by reflexion on the thin laminæ, became like a kind of sky, coloured by that tint alone, which was thrown on it. In this manner the following results were obtained:
(1.) Each kind of simple light produced rings of its own colour, both by reflexion, and by transmission.
(2.) In each case, the rings were separated by dark intervals, which made them much more distinct than in the original experiment, and caused many more to be discerned. They were more and more crowded together as their distance increased from the central spot.
