the return or progressive motion of a moving body, occasioned by some obstacle which hindered it from pursuing its former direction.

Circular Instrument of Reflection, an instrument for measuring angles to a very great degree of accuracy. It was invented by the celebrated astronomer Mr Tobias Mayer of Gottingen, principally with a view to do away the errors of the divisions of the limb; and has since been much improved by the Chevalier de Borda, and M. J. H. de Magellan. This instrument is particularly applicable to the measuring of the distances of the heavenly bodies, and was used by the French in their part of the operation for determining the difference of meridians of Paris and Greenwich. For the description, rectification, and use of this instrument, see Navigation.

Reflection of the Rays of Light, in Catoptrics, is their return, after approaching so near the surface of bodies as to be thereby repelled or driven backwards. For the causes of reflection, see Optics Index, at Rays of Light, and Reflection of Light, &c. For the application of the doctrine of reflection to mirrors, see Optics. See also Mirror, Burning-Glass, and Glass Grinding; and for the coating or foliating of mirrors, see the article Foliating of Looking-glasses, &c. See also Telescope.

Reflection of Heat, see Chemistry, No 170.

Reflection of Cold. For an account of this curious phenomenon, see also Chemistry, No 272.

It has been generally supposed that this fact was first noticed by Professor Picquet of Geneva; but we have been informed from good authority (for we have not yet Reflection, yet had an opportunity of seeing the book), that the same fact is distinctly mentioned by Baptista de Porta in his *Magia Naturalis*.

Reflection is also used, figuratively, for an operation of the mind, whereby it turns its view backwards as it were upon itself, and makes itself and its own operations the object of its disquisition; and by contemplating the manner, order, and laws, which it observes in perceiving ideas, comparing them together, reasoning, &c. it frames new ideas of the relations discovered therein. See Metaphysics.

Reflectors for Light-Houses, have of late years been very successfully adopted instead of coal fires. They are composed of a number of square pane glass mirrors, similar to those which, it is said, were employed by Archimedes in setting fire to the Roman fleet at the siege of Syracuse. The mirrors are an inch square, and are disposed close to each other in the concave of a parabolic segment, formed of stucco, or any other substance which retains them in their place. Stucco, however, is found to answer sufficiently well, and is employed in the reflectors of all the light-houses which have been erected round the coast of Scotland.

The parabolic moulds are from three to five or six feet in diameter, and in the centre of each there is a long shallow lamp of tin plate, filled with whale oil. There are six cotton wicks in each lamp nearly contiguous to each other, and so disposed as to stand in no need of trimming for the space of six hours. The light is reflected from each mirror spread over the concave surface, and is as it were multiplied by the number of mirrors. Tin plate covers the back of the stucco moulding, from which a tube immediately over the lamp proceeds to the roof of the light-room, and answers the purpose of a funnel, through which the smoke passes without suffusing the face of the mirror. The light-room is a lantern of from eight to twelve sides, entirely made of glass, fixed in frames of cast iron, and roofed with copper. The reflectors with their lamps are placed on circular benches passing round the inside of this lantern, at about 18 inches from the glass frames, so that the concave surfaces of two or three of the reflectors front every point of the compass, and throw a blaze of light in all directions.

There is a hole in the roof, directly over the centre of the room, through which all the funnels pass, and by which fresh air is also conveyed to the lamps. This light-room is fixed in such a manner on the top of a round tower, that no weather can move it; and the number of the reflectors, and the height of the tower, are greater or less, according as the light is intended to be seen at a greater or less distance.

It has been proposed to make the concave surface of the parabola one speculum of metal, instead of covering it over with a number of plain glass mirrors, or to diminish the size of each mirror, if they are preferred to the metallic speculum. It must be obvious to every man who knows anything of optics, that either of these alterations would be improper. The brightest metal does not reflect so much light as plain clear glass, and if the size of the mirrors was diminished, the number of joinings would be increased, in each of which some light is lost.

A man wholly guided by theory, would be ready to condemn light-houses of this description; because a violent storm will shake the firmest building, which, in his opinion, would throw the whole rays of light into the air, and thus mislead the bewildered mariner. Experience, however, shows, that such apprehensions are groundless, and that light-houses with lamps and reflectors, are in all respects preferable to those with fires burning in the open air. They are less expensive; they give a more brilliant light, and are seen at a greater distance, and cannot be obscured by smoke, or driven down on the lee-side by the most violent wind. If to this we add, that the lamps do not stand in need of trimming so often as fires require fuel, and that the light-man is never exposed to the weather, we must allow that light-houses with reflectors are not so liable to be neglected in stormy weather as those with open fires, which alone must give the former a preference over the latter.

It has been asserted, and particularly stated, in the supplement to the third edition of this work, that Mr Smith of Edinburgh, the principal, and we believe now the sole contractor for managing and keeping in repair the light-houses round the coast of Scotland, is the first who conceived the idea of illuminating light-houses by means of lamps and reflectors. We do not understand that Mr Smith himself ever claimed the merit of this invention; but it appears that reflectors, such as are described above, were invented by Mr Ezekiel Walker of Lynn Regis, who says, in a letter dated October 1801, and addressed to the editor of the Monthly Magazine*, that such reflectors were made and fixed up under his direction, in a light-house on the coast of Norfolk, in the year 1779; and adds farther, that in the year 1787, at the request of the trustees appointed by act of parliament for erecting four light-houses on the northern coast of Great Britain, he instructed Mr Smith in this method of constructing light-houses. Mr Walker's statement of the fact is confirmed by a letter from Mr Grieve, then lord provost of Edinburgh, who informs Mr Walker that the trustees had agreed to pay the premium required for communicating the invention, and that Mr Smith was engaged to go to Lynn Regis to receive instructions from Mr Walker in the method of constructing the new reflectors.