Page:Optics.djvu/170

 (9.) The simple rings of each colour were least when the rays passed perpendicularly through the lamina of air, and increased with the angle of incidence.

These observations explain completely the more complicated phænomenon of the rings formed by the natural light, for this light consisting of different coloured rays mixed together in definite proportions, when a beam of this mixture falls on the thin lamina of air between the glasses, each kind of simple light forms its own rings by itself, according to its own peculiar laws, and as the diameters of these rings are different for the various kinds of light, they are sufficiently separated from each other to be distinguished. However, this separation is by no means so perfect as in observations made with simple rings, because the rings of different colours encroach a little on each other, so as to produce that infinite diversity of tints that the experiment shows. But, though this successive superposition of the simple rings is really the key of the phænomena, one cannot be very sure of the fact without having measured exactly the absolute magnitudes of the diameters and breadths of the rings, formed by the different coloured rays; for when these results are once known, it can only be a simple arithmetical problem to find the species and the quantity of each colour that may be reflected or transmitted at each determinate thickness; and consequently, if the effects of the composition of all these colours be calculated by the rules which Newton has given in his Optics, it will be easy to deduce, with perfect accuracy, the numerical expressions of the tint and intensity of colour which must exist at each point of the compound rings, which may then be compared with experiment. In a word, we have as yet only a suspicion, a probable one no doubt, of the cause of our phænomena; accurate measurements are necessary to convert that probability into certainty.

This is just what Newton did. He measured the diameters of the simple rings of the same order, both at their inner and outer edges, taking successively the various colours of the spectrum, from the extreme violet to the deepest red; afterwards, according to his usual method, he took care to connect these results by a mathematical law, which might represent them with sufficient accuracy. Then comparing the squares of the diameters, he deduced the