the upper surface was plain and polished, but the under surface either
plain, concave, or convex. These were placed before a window, in
such a position that the light fell upon the lens at about 30° from
the perpendicular, and was received by the eye, at an equal elevation, on the opposite side.
Instead of the surface of glass, a metallic surface was next substituted; and the same lens placed upon it, gave the same appearance of similar concentric rings.
Moreover, it is not necessary that the surface of the under substance should be plain. It may be either concave or convex; so also may the upper surface, in contact, be either convex or concave, provided that when a concave surface is applied to another that is convex, the radius of concavity be greater than that of the convexity to which it is applied.
The second section treats of seeing the same rings by transmission, which. of course, admits the same variety in the forms of the surfaces in contact, but will not allow either of them to be metallic.
The third section distinguishes the several images, of any object, that are reflected from the different surfaces of several plates of glass, laid one upon another, on account of the use that may be made of these images, in assisting to discern the complicated phenomena produced in succeeding sections.
In the fourth section a second series of rings is produced, by placing the lens upon a piece of looking-glass, which occasions the primary set to be seen a second time by reflection. But as this is less bright, the primary set must first be obscured, by bringing the second reflected image of a pen-knife, or other pointed body, over it. In this case there are three images of the pen-knife. The second obscures the primary set of rings; the third shows them to the greatest advantage.
The same varieties of contact which were found to make one set of rings, may, of course, be applied to make a secondary set, if there be a reflection beneath sufficiently bright to render it visible.
The fifth section treats of three sets of rings, produced by increasing the number of reflecting surfaces, as when a slip of glass is interposed between the lens and the looking-glass of a former experiment, or when the lens, laid upon two slips of glass, is placed on a plain metallic reflector.
The sixth section pursues the same complicated appearance, as far as four sets of rings, and shows how they may be discerned, by means of the reflected images of the pen-knife.
In the seventh section the size of rings is considered, so far as it depends on the curvature of the surfaces; but Sir Isaac Newton having already treated this part of the subject at large, Dr. Herschel does not think it necessary to enter further into it.
In the eighth section, the species of contact requisite for exhibiting the rings is mentioned, the size of them being considerably affected by pressure. They grow larger when the two surfaces that form them are pressed closer together, and diminish in proportion as the
