from the glass; and even if it did, some one of the oils with which it was in contact in the foregoing experiments must have had the same refractive energy, and must thus have deprived it of its power to develope the periodical tints. In the hope of unravelling this mystery, I took two prisms of glass cut out of the same plate, and which gave fine periodical colours with castor oil. By the aid of screws I pressed the bases of the prisms into optical contact: at great incidences the light was yellow; and by diminishing the inclination of the ray it became gradually orange and deep red when it vanished, no light being visible at smaller angles of incidence. In this experiment the surfaces of the two films, if they do exist, were brought into optical contact, so that we ought to have had orders of colours corresponding to a film of twice the thickness. But even if such a film could be supposed to exist invisibly on the glass, it could not afford any explanation of the splendid colours which are exhibited when the solid is a crystallized mineral, and where its tint is related to its axis of double refraction. That some unrecognised physical principle is the cause of all these phænomena, will appear still more probable when I submit to the Society a paper on the very same periods of colour produced at similar angles of incidence, by the surfaces of metals and transparent solids when acting singly upon light.

The action of the surfaces of crystallized bodies presents many remarkable phænomena, in the investigation of which I have been long occupied. The results to which I have been led will form the subject of two communications. The first will treat of the action of the surfaces of bodies as an universal mineralogical character, with the description of a lithoscope for discriminating minerals. The second will contain an inquiry into the influence of the doubly refracting forces upon the ordinary forces which reflect and polarise light at the surfaces of bodies. My early experiments on this subject are recorded in the Phil. Trans. for 1819, but I have resumed the inquiry, and have obtained results of considerable interest.

Allerly, February 2nd, 1829.

XVIII. On the reduction to a vacuum of the vibrations of an invariable pendulum. By Captain EDWARD SABINE, of the Royal Artillery, Secretary of the Royal Society. Communicated by Dr. THOMAS YOUNG, Secretary of the late Board of Longitude.

Read March 12 and 19, 1829.

THE 128th number of Professor SCHUMACHER'S Astronomische Nachrichten, published in January 1828, contained an announcement from M. BESSEL, that he had found the theory incorrect, according to which it has been customary to reduce the vibrations of a pendulum in air, to the corresponding vibrations in a vacuum: the incorrectness consisting principally, in no provision having been made in the theory, for the expenditure of a part of the moving force, on the particles of the air set in motion by the pendulum in its vibration.

On the arrival in London of the number of the Astronomische Nachrichten containing this announcement, a proposal was made to the late Board of Longitude, to submit the question, of the reduction of the vibrations to a vacuum, to the test of the most direct experiment; by the construction of an apparatus, in which a pendulum might be alternately vibrated in air of full atmospheric pressure, and in rarefied air approaching nearly to a vacuum. The expense of the proposed apparatus was estimated at 257.; which sum the Board of Longitude, at the recommendation of the President of the Royal Society, and of Dr. YOUNG Secretary of the Board, was pleased to allot for that purpose. Mr. NEWMAN, who was employed to make the apparatus, gave great attention towards accomplishing it in the best manner; and to his care in respect to expense it is owing, that the cost has but very little exceeded the estimate. How well it has answered its intended purpose will be best collected from the experiments themselves.

The apparatus is represented in Plate VI, which may be referred to for the particular dimensions. It consists, generally, of six pieces, exclusive of the iron