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

Circular Instruments 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 Optics, is their return, after approaching to 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 polishing of mirrors, see the article Follating 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 Pictet 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.