In the history of-optics, particular attention is due to his theory of light, which. was first Communicated to the Academy of Sciences of Paris, in 1678, and afterwards published, with enlargements, in 1690.
Light, according to this ingenious system, consists in certain undulations communi cated by luminous bodies to the etherial 'fluid which fills all space. This fluid is composed of the most subtle matter, is highly elastic, and the undulations are propagated through it with great velocity in spherical superficies proceeding from a centre. Light, in this view of it, differs from that of the Cartesian system, which is supposed to be without elasticity, and to convey impressions instantaneously, as a staff does from the object it touches to the 'hand which holds it.
It is mot, however, in this general view, that the ingenuity of the theory appears, but in its application to explain the equality of the angles of incidence and reflection ; and, most of all, the constant ratio which subsists betweenthe sines -of the angles of incidence and of refraction. Few things are to be met with more simple and beautiful than this last appli cation of the theory ; but that which is most remarkable of all is, the use made of it to ex plain the double refraction of Iceland crystal. This crystal, which is no other than the calcareous spar of mineralogists, has not only the property of refracting light in the usual manner of glass, water, and other transparent bodies, but it has also another power of re fraction, by which even the rays falling - perpendicularly on the surface of the crystal are turned' out of their course, so that a double image -is formed of all objects seen through these crystals. This property belongs- not only to calcareous spar, but, in a greater or less degree, to all which are both crystallised and transparent.
The common refraction -is explained by Huygens, on the supposition, that the umiak dims in the luminous fluid ate propagated in the form of spherical waves. The double •re fraction is explained on the supposition, that the undulations of light, in passing through the calcareouaspar, assume a spheroidal form ; and this hypothesis, though it does not ap ply with the same simplicity as the former, yet admits of such precision, that a proportion of the axes of the •pheroid• may be 'assigned, which will account for the precise quantity of- the extraordinary. refraction, and. for all the phenomena dependent on it, which Huy
gens had studied with-great care, and had reduced to the smallest number of general facts. That these spheroidal undulations actually exist, he would, after all, be •a bold theorist who•should affirm -; but that the supposition of their existence is • an accurate expression of the phenomena of double refraction, cannot be doubted. When one enunciates the hy. pothesis of the spheroidal undulations, he, in fact, expresses in a single sentence all the phenomena of double refraction. The hypothesis is therefore the means of representing these phenomena, and the laws which they obey, to the imagination or the understanding, and there is, perhaps, no theory in optics, and but very few in natural philosophy, of which more can be said. Theory, therefore, in this instance, is merely to be regarded as the ex. pression of a general law, and in that light, I think, it is considered by La Place.
To carry the theory of Huygens farther, and to render it quite satisfactory, a reason ought to be assigned why the undulations of the luminous fluid are spheroidal in the case of crystals, and spherical in all other cases. This would be to render the generalization more complete ; and till that is done, and a connection clearly established •between the structure of crystallized bodies, and the property of double refraction, the theory will re main imperfect. The attention which at present is given to this most singular and inter esting branch of optics, and the great number of new phenomena observed and classed under the head of the Polarisation of Light, make it almost certain that this object will be either speedily accomplished, or that science has here reached one of the immoveable barriers by which the circle of human knowledge is to be for ever circumscribed.