SPECULUM. This name is frequently given to a mirror used for any scientific pur pose, as in a reflecting telescope. If a pencil of rays diverge from a radiant point in the axis of a concave speculum of a spherical form, all the rays will, after reflexion, converge nearly to a certain point in the same axis, at which the image of the radiant point is said to be formed. This point is erdled the Focus. None of the rays are, in strictness, reflected to the focus, but all those which are reflected from the mirror at points very near its intersection with the axis fall extremely near it. The image in a convex mirror is always uprigbt, and in a concave one always inverted, except when the object falls between the principal focus (or middle point of the radius) and the mirror.
The best composition for the metal of re fleeting telescopes is a subject which has been ' much investigated. About 70 different mix. trues were tried by the Rev. John Edwards, ; the particulars of which are stated in the Nautical Almanac' for the year 1787; lie found copper 32, tin 15, brass 1, silver 1,white oxide of arsenic 1, to form an alloy which was . the whitest, hardest, most reflective, and took ; the highest polish.
r In the Philosophical Transactions' for 1 1810, Lord Rosse, then Lord Oxmantown, s published an account of a speculum 3 feet t diameter which he had then executed ; and a from this we learn that after many trials tc The most convenient method of obtaining the specific gravities of fluids is by means of what chemists call a thousand-grain bottle.' This is a bottle of a globular form, with a ground glass stopper, so adjusted as to contain exactly 1000 grains of distilled water, at the temperature of 60° Fahr., and accompanied by a weight, which is an exact counterpoise for the bottle when thus filled. In order to determine the specific gravity of a fluid by this means, it is simply necessary to fill the bottle with that fluid at the temperature of CO°, and place it, together with the adjusted weight, in the opposite scales of a delicate balance ; then the number of grains which it will be found necessary to add to one of the scales, in order to produce equilibrium, will be the difference between the specific gravity of the fluid and•that of water, taken at 1000.