Mirrors of this kind, with a moveable- focus, were re garded by Buffon as of great use for measuring the effects of the solar rays, when concentrated into foci of different sizes. As the quantity of incident light and heat is nearly the saute to whatever curvature the glass is successively bent, we might thus determine the size of focus by which a maximum effect was produced.
Baron likewise made a number of concave mirrors, by bending plates of glass on moulds of a spherical form. Some of these were so large as 3, 4, 4 feet 6, and 4 feet 8 inches in diameter ; but the utmost care is requisite in the formation of those of such a large diameter. After these glasses were moulded to the proper shape in ap propriate furnaces, their concave and convex sides were carefully ground so as to be perfectly concentric, and the convex side was afterwards silvered by M. de Bernieres. Out of twenty-four mirrors of this kind which Buffon had moulded, he was able to preserve only three, the rest having broken, either by exposure to the air, or in the operation of grinding. One of these three, which was 46 inches in diameter, was presented to the king of France, and was regarded as the most powerful burning mirror in Europe. The other two were 37 inches in diameter, and one of them was deposited in the Cabinet of Natural History in the Jardin du Rol. I3uffon con centrated the rays of the moon by means of the mirror of 46 inches diameter, but, though his thermometer was very sensible, no heat was perceived.
The great defect in the large burning mirror of Buf for., and, indeed, in every- combination of plain mirrors, is the difficulty of adjusting each mirror, so as to be always perpendicular to the straight line which bisects the angle formed by the ray of the sun, and the line which joins the mirror and the focus of the paraboloid. The neces sary motion might easily be given to all the elementary mirrors, by attaching each of them to a machine resem bling the IIeliostata of Gravesende ; but the dilatation of its parts, and the shakings to which it would inevitably be exposed, might occasion an agitation in the different images, which would greatly diminish the power of the machine. The only method, therefore, of combh ing
plain mirrors, is to confide each of them to an individual, properly instructed, to keep it in such a position as to reflect the solar rays to a determinate point, and to vary this position according to the motion of the sun. M. Pey rard, however, properly observes, that though this might be easily done by three or four persons, yet if 100 or 200 individuals were thus employed, none of them could dis tinguish the image of his own mirror from that of the rest ; and, therefore, if a single image were removed from the focus, each of the mirror-holders would natu rally wish to know if it was his, and a general agitation and confusion of the images would be the consequence. In order to remedy this inconvenience, M. Peyrand pro poses to furnish each mirror with suitable apparatus, so that, when it is once directed to the point where the images are to be collected, the individual who has the charge of it can always adjust it to the motion of the sun, without looking beyond the apparatus itself.
The apparatus with which each mirror is connected, is represented in Plate CV. Fig. 9, where AB is a com mon telescope, with only one tube, containing the object glass at B, and the eye-glass at A. This tube is move able on its axis, between the two collars CC, CC', which are fixed to a piece of metal DD. This piece of metal is supported on a stand like a common telescope, having a vertical and horizontal motion, by which the axis of the telescope may be directed with facility to any given point. The axis of the instrument is marked out by the inter section of a pair of cross-wires placed in the anterior fo cus of the eye-glass ; and when this point of intersection is directed to any object, the whole instrument is kept steady in its place by the screws F and G, the former of which prevents any motion in a vertical direction, and the latter in a horizontal direction. From the middle of the tube AB rises a cylindrical piece of metal MM, and upon the eye-glass extremity a branch of iron HMI, wrought square, is fixed firmly in a direction parallel to the axis of the cylindrical piece MM.