He next examines which are the reflecting surfaces. by means of
certain scratches, and other defects.
In the 25th and 26th section he finds, by means of similar defects, that the surfaces in contact are alone concerned in the formation of rings; and in the 24th and 28th section he discovers, by various irregular surfaces which he employs, as [st and 4th surfaces of two glasses in contact, that these are not concerned in the production of rings. And in the 27th section he observes, that the colour of the under glass does not affect the primary set of rings.
The results of the foregoing experiments are,—
I. That only two of the surfaces are essential to the formation of concentric rings.
II. That these two must be of a certain regular construction, so as to form a central contact.
III. That rays, from one side or the other, must pass through one of the surfaces at or near the point of contact to the other surface, and be reflected from it.
And IV. That in all these cases a set of rings will be formed, having their common centre in the point of contact.
The cause of these phenomena, Dr. Herschel says, must be either in the nature of the rays themselves, or in the surfaces; and if it can be shown that the disposition to fits of easy transmission and reflection does not exist, a proposition of accounting for them by modifications occasioned by the surfaces, he thinks, will find a ready admittance.
In section 30, he shows that the word transmission will not apply to the ease where rings are produced by placing a lens upon a metallic surface, and wishes to substitute the word absorption.
In section 31, Dr. Herschel contends that a plate of air, of the thinness which is supposed sufficient, will not give coloured rings, because in a case of circumferential contact, where a concave surface was applied to one that was convex, of very little larger radius, he could not perceive any appearance of colour.
In section 32, he places a piece of plain glass, four tenths of an inch square, on a concave glass mirror of 10 feet focus, but could observe no rings or colours.
In section 33, he does not find that a secondary set of colours, produced in the usual way, is altered by being seen through a wedge of air, occasioned by the interposition of card between the edges of two slips of glass,
And finally, in section 34, Dr. Herschel could discern no colours when two slips of plain glass, two inches long, were in contact at one extremity, and distant only Trio-Vth of an inch at their other extremities; although in the first half-inch from their contact, the several distances which Sir Isaac Newton considers as capable of producing ten successions of colours, must have occurred.
Dr. Herschel therefore infers, that the rays of light have no disposition to be alternately reflected and transmitted at certain intervals of space; but the examination of the various modifications that light
