145
(3.) The dark intervals which separated the bright rings seen by the reflected light, were bright rings themselves by the transmitted rays, and they were separated by dark intervals answering to the former rings. However, those intervals were not exactly black, because the reflexion on a thin lamina of air is far from being perfect, even in the most brilliant part of the reflected rings; and the same thing may be observed of all thin transparent plates of any substance whatever.
(4.) In observing the luminous reflected rings, Newton remarked, that they were not simple geometrical lines, but that each of them occupied a certain space, in which the brightness diminished gradually each way from the middle.
(5.) Measuring the diameters of the reflected rings at their brightest part, he found that for each particular kind of rays, the squares of the diameters followed the arithmetical progression of the numbers 1, 3, 5, 7, &c.; consequently, the thicknesses of the lamina, which are as the squares of their diameters, were in that same progression.
When the glasses were illuminated by the brightest part of the spectrum, which is between the orange and yellow, the diameter of the sixth ring was found to be the same as that of the brightest part of the corresponding ring in the experiment made in full daylight.
(6.) The diameters of the dark rings being likewise measured, he found that their squares, and consequently, the thicknesses of the air below them followed in the progression, 2, 4, 6, 8, &c.
(7.) By other measurements, he discovered that the brightest parts of the transmitted rings answered to the darkest parts of the intervals in reflexion, and vice versâ, the darkest parts here were the brightest in the other case, so that the thicknesses of air which transmitted the bright rings, and those which gave dark intervals, were respectively as 2, 4, 6, 8, &c. and as 1, 3, 5, 7, &c.
(8.) The absolute diameters of corresponding rings of different colours were different, as were also their breadths, both these dimensions being greatest for the extreme red rays, and least for the violet.
