Page:Optics.djvu/171

 thickness of the lamina of air at each edge of the observed rings. Similar measurements effected with respect to the different orders of rings, formed by one simple colour, proved to him, that the intervals of thickness, throughout which reflexion took place, were sensibly equal to those which allowed transmission, at least when the light was incident perpendicularly. Thus, designating generally by t the thickness of the air at the beginning of the first lucid ring, for any simple colour, that ring ended at the thickness 3t, and therefore occupied an interval of thickness equal to 2t. Then came the first dark ring, occupying an equal interval 2t; then a second lucid ring from 5t to 7t, and so on.

Combining this law of succession for the different orders, with that of the distribution of the various tints of the same order, one easily conceives that a single absolute thickness, measured at the beginning, the middle, or the end of any ring formed by a simple colour, is sufficient to calculate the value of the first thickness t, relatively to that colour, and thus all thicknesses of the several rings of each colour may be determined.

In this manner Newton, measuring the thickness represented by 2t for the different simple rays, in vacuo, in air, in water, and in common glass, found their values as shown in the following Table, where they are expressed in ten thousandth parts of an inch.