Page:Optics.djvu/167

 layer of water, of alcohol, of ether, or any other evaporable liquid, spread on a black glass, produces similar colours, when sufficiently attenuated; they may be observed also on soap bubbles, and on blown glass, when thin enough.

In whatever manner, and under whatever circumstances these rings are formed, the succession of their colours from the central dark spot is invariably the same; the only difference perceptible is in their brightness, which varies with the refracting power of the lamina, and in their form, which depends on the law by which the thickness of the lamina is regulated in different parts. In fact, for any one substance, the colour reflected at any point depends on the thickness of the lamina, and the incidence under which the reflexion takes place.

So far we have supposed the colours of the lamina to be seen only by reflexion; if it be placed between the eye and the light, concentric rings will again be observed similar to the others in form, but not in colour, and fainter, surrounding a bright spot.

This might naturally be expected, for when the incident light is decomposed, so as to give coloured rays in the reflexion, those transmitted must of course be also coloured, and the one set must, in fact, be complementary to the other, that is, both together would produce white.

It follows from all this, that to discover the laws of these phænomena, the best method is to study them in cases where the variation of thickness is regular and known. This is what Newton did; and he conducted his researches with a careful nicety, which could be owing only to the importance which he foresaw would be attached to the consequences of them.

He formed the rings by placing a convex glass of small curvature on a piece of perfectly plane glass; then the thickness of the lamina of air increasing symmetrically in all directions from the point of contact, the rings were perfectly circular round the dark spot formed at that point.