OPTICS, PRACTICAL, is that part of science which applies the physical properties of LIGHT and the mathematical laws of Orrres to the construction of useful optical instruments. By the former we determine the constants necessary to render the formulae of the latter convertible into numbers. The refractive and dispersive indices peculiar to transparent media are constants of this nature, and the instruments adapted to the easy vision of near or distant objects, to great or small objects, and to other optical purposes, are, according to the plan of this work, described under their proper heads. [CAmEne LECIDA ; HELIOSTAT; MICROSCOPE; TELESCOPE: &C.1 The refractive indices of transparent and semi-transparent media have been a subject of research to many experimenters, and were con siderably advanced by Newton. (Newton's Optics.') The additional properties of light discovered since his time have enabled philosophers to calculate to a far greater degree of accuracy the indices both of re fraction and dispersion than was then practicable.
The theory of achromatism, or the method of correcting the aberra tions of the rays of light, has been pursued by Euler, D'Alembert, Herschel (Sir J.), and many others ; but the earliest successful con struction was made by Mr. Hall, in 1733. The same was effected in 1757 by Dollond, whose labours, together with that of his son, gave a great impulse towards the complete accomplishment of an object of which Newton seems almost to have despaired. In the same career of late years we must distinguish Fraunhofer, of Benediktbeuern, in Bavaria, who obtained at an early age from the French Academy the prize for the actual construction of achromatic glasses. Not only were the necessary manual operations conducted by himself with patience and the minutest attention to all the practical details of the quality of his glasses and the accuracy of grinding and polishing, but be had also the merit and advantage of observing the dark lines• which cross the prismatic spectrum, and which are of the greatest utility in determining the indices above alluded to. From the nature of the composition of
the glasses, some of his forest telescopes have of late become consider ably tarnished, particularly those in exposed situations. This could not have been easily foreseen, and many of his telescopes remain still in good condition.
The first account of Fraunhofer's remarkable optical discoveries is given in a paper which he published in the Memoirs of the Academy of Bavaria' for 1814-15. By means of a theodolite furnished with a telescope, he measured the distances of the principal lines ; and by applying a photometer to the different coloured rays, he drew a curve, the ordinates of which express the illuminating powers of the several rays. To these researches he soon afterwards added some beautiful experiments on the diffraction of light, an account of which he published at Munich, and they also appeared in an abridged form in the Bibliotheque Universelle,' January, 1822. It is believed that his close application to those and similar researches accelerated his death, which followed soon after.
The accurate determination of the refractive and dispersive indices has also been pursued with great success in this country, and simul taneously by some of the following distinguished men :—Dr. Thomas Young, Dr. Brewster, Dr. Wollaston, Sir. J. Herschel, Prof. Faraday, Prof. Powell, &c. ; and in France, by Biot, Dulong, &c. ; some of the results of whose experiments, in a very compressed form, are given in the two following tables :—