10. NOW it is evident, that these colours arose at the top of the bubble, as it grew by degrees thinner and thinner: but what the express thickness of the bubble was, where each of these colours appeared upon it, could not be determined by these experiments; but was found by another means, viz. by taking the object glass of a long telescope, which is in a small degree convex, and placing it upon a flat glass, so as to touch it in one point, and then water being put between them, the same colours appeared as in the bubble, in the form of circles or rings surrounding the point where the glasses touched, which appeared black for want of any reflection from it, like the top of the bubble when thinnest[302]: next to this spot lay a blue circle, and next without that a white one; and so on in the same order as before, reckoning from the dark spot. And henceforward I shall speak of each colour, as being of the first, second, or any following order, as it is the first, second, or any following one, counting from the black spot in the center of these rings; which is contrary to the order in which I must have mentioned them, if I should have reputed them the first, second, or third, &c. in order, as they arise after one another upon the top of the bubble.
11. But now by measuring the diameters of each of these rings, and knowing the convexity of the telescope glass, the thickness of the water at each of those rings may be determined with great exactness: for instance the thickness of it, where the white light of the first order is reflected, is about 3⅞ such parts, of which an inch contains 1000000[303]. And this measure gives the thickness of the bubble, where it appeared of this white colour, as well as of the water between the glasses; though the transparent body which surrounds the water in these two cases be very different: for our author found, that the condition of the ambient body would not alter the species of the colour at all, though it might its strength and brightness; for pieces of Muscovy glass, which were so thin as to appear coloured by being wet with water, would have their colours faded and made less bright thereby; but he could not observe their species at all to be changed. So that the thickness of any transparent body determines its colour, whatever body the light passes through in coming to it[304].