Zeiher also constructed burning glasses, by mak ing a concave frame of wood, and covering the concave surface with a paste made of flower, chalk, &c. till it had the requisite degree of curvature. A number of pieces of silverized glass mirrors, about half an inch square, were then fixed upon the concave side, so as to constitute a polygonal reflecting surface.
About the same time that Zeiher was occupied with these pursuits, the celebrated Buffon was engaged in the improvement of burning instruments. This distinguish ed naturalist directed the whole energy of his powerful mind to this curious subject ; and when we say that he has in a great measure exhausted it by the ingenuity of his contrivances, we are not detracting much from the merit of his successors.
Before Buffon began to construct his great mirror, he made a number of preliminary experiments, which are well worthy of being recorded. lie found that silvered glass reflected light more powerfully than the best polish ed metals, even than those which are employed for the specula of reflecting telescopes ;—that at short distances, such as four or five feet, only one half of the light was lost by reflection ;—that almost no light was lost by being transmitted through 100, 200, and 300 feet of air ;—that more of the light of candles was lost by reflection than the sun's light, owing to the greater obliquity of the in cident rays ;—that the image reflected from a plain mir ror six inches square, is at a short distance also six in ches square, hut afterwards it augments, and then grows deformed, till it at last becomes round at greater distan ces ;—that a plain mirror six inches square loses its square figure at the distance of 60 feet, and one of a foot square at the distance of 120 feet ;—that a lens 32 inches in diameter and six inches focal length, having the dia meter of its focus 8 lines, melted copper in less than a minute, while a small lens 32 lines in diameter, with a focal length of 6 lines, and its focus or 4 of a line, was scarcely capable of heating copper, though the two lenses had theoretically the same burning power, and that a large focus was therefore preferable to a small one, in the fusion of metals. After having determined these preli minary points, Buffon constructed, with the assistance of Passemant, a very powerful mirror, which we shall now proceed to describe.
Tdis mirror, which we have represented in Plate CV. Fig. 1., was at first composed of 168 pieces of plain sil vered glass, six inches by eight, and having an interval of four lines between each, for the purpose of allowing them a free motion in every direction, and also of per mitting the observer to see the place to which the images were to be thrown. These pieces of glass were mount ed in an iron frame, and each of them was so fitted up with screws and springs, that a motion could be given to them in any direction, so that the images reflected from all the mirrors might be easily thrown upon the same spot. The machinery for giving motion to each mirror is re presented in Fig. 2, 3, and 4. The greatest difficulty ex perienced in the formation of this mirror, was to obtain glasses sufficiently plane. Bufron selected those that gave the roundest image of the sun at the distance of 150 feet ; and he had to examine more than 500 before he could procure 168 that answered his purpose. With this instrument the following results were obtained.
March 23. 1747, Buffon set on fire, at the distance of 66 feet, a plank of tarred beach wood, with 40 mirrors only. In this experiment, the mirror was not put upon its stand, and was therefore very disadvantageously placed, forming with the sun an angle of 20° of declina tion, and another of more that 10° of inclination.
March 23. 1747. The mirror being still more disad vantageously placed, a plank tarred and sulphured was set on fire at the distance of 126 feet, with 98 mirrors.
.4pri/ 3. 1747. At 4 o'clock in the evening, when the sun's rays were weak, and his light very pale, and when the mirror was mounted upon its stand, a slight inflam mation was produced upon a plank covered with wool cut into small pieces (lame hachee) at the distance of 138 feet, with 112 of the mirrors.
i/pri/ 4. 1747. At 11 o'clock in the morning, when the sun was very pale and obscured with vapours and light clouds, 154 mirrors at the distance of 150 feet, made a tarred plank smoke to such a degree in two mi nutes, that it would have been inflamed, had not the sun quickly disappeared.