In order to at oid the great thickness of glass at the centres of large convex Icnses, has proposed a very ingenious contrivance. Instead of making the burning lens of one piece of glass, he proposes to lorni it of three concentric circular pieces resting upon each other. Thus if the whole diameter of the lens is to be 24 inches, which would require a central thickness of three inches if it were of solid glass, the middle part will be a lens 8 inches in diameter, with a thickness of one inch. This lens is inserted in the middle of a circular zone, whose diameters are 8 and 16 inches, and this circular zone is again inserted in the middle of another circular zone, whose diameters are 16 and 24 inches. The surfa ces of the lens and of the two zones arc all ground to the same radius ; so that when they are placed together, the solar rays will be refracted to one focus, in the very same manner as if they had fallen upon a lens 24 inches in di ameter. The great advantages which are gained by this construction, is the diminution of the quantity of glass, as it does not require half as much as is necessary in lenses of one piece. In consequence of this diminution of thick ness, the power of the lens is remarkably increased. The rays which fall upon the central parts, instead of being absorbed by the great mass of glass through which they had to pass, will he transmitted through the lens of 8 inches, and will be twice as powerlul as if they had been refracted by a similar portion of a solid and continuous lens.
The next burning lens of any magnitude was con structed by M. Bernieres for M. Trudaine de Mon tigny, an honorary member of the Royal Academy of Sciences. This gentleman, whose liberality and zeal de serve to be recorded, engaged to be at the expellee of a large burning glass, formed under the direction of seve ral commissioners named by the Acatb.my. This lens consisted of two spherical segments 8 feet radius and 8 lines thick. The lenticular cavity was 4 feet in diame ter, and 6 inches and 5 lines thick at the centre, and was filled with spirits of wine, of which it held no less than 140 pints. The focal length of a zone at the circum ference, about 6 or 7 lines broad, was 10 feet 0 inches and 6 lines, the focal length of a portion at the centre, about 6 inches in diameter, was 10 feet 7 inches and 5 lines, and the diameter of the focus was 141 lines. When the whole surface was covered, except a zone at the cir cumference of 6 or 7 lines, the following were the foci of the diffemn rays : Fret. Inch. Lines.
Violet 9 6 4} from the centre of the lens.
Blue 9 7 10.1 Yellow 10 2 3 Orange 10 2 10 Red 10 3 111 The following experiments were made in October 1774, in the Jardin de l'infante, by M. i11. 'Trudaine, Macquer, Cadet, Lavoisier, and Brisson, the commis sioners appoi..ted By the Academy.
1. The burn ing power of the anterior half of the lens was much grt:,ter than that of the exterior half.
2. On the 5th of October, after mid-day, the sky not being very clear, two farthings, placed upon charcoal, were completely melted in half a minute.
3. In order to melt forged iron, it was found necessary to concentrate the lays by a second lens 81 inches dia meter, 22 inches 8 lines in focal length, and placed at 8 feet 7 inches from the centre of the great lens. At this place, the cone of rays was 8 inches in diameter, and the burning focus, now reduced to 8 lines in diameter, was foot from the small lens.
4. In the focus of the small lens, upon a piece of hol low charcoal, small pieces of forged iron were placed, which were instantly melted. After fusion, the metal bubbled up, and fumed like nitre in fusion, and then sent off a great number of sparks. This effect (which was observed during the expel intents with Tschirnhausen's lens,) always took place after the fusion of iron, forged iron, or steel.
5. In order to try the effect upon greater masses, pieces of forged iron, and the end of a nail, were exposed to the focus, and were melted in 15 seconds. A piece of nail, 5 lines long and of a line square, which was added to the rest, was instantly fused, and the same was the case with a screw that bad a round head, and was 8 lines in length.
6. Some days afterwards, a bar of steel, 4 inches long, and 4 lines square, was exposed, so as to receive the focal image upon the middle of its length. This part was com pletely melted it 5 minutes, after having begun to run at the end of the second minute.
7. Platina, in grains, appeared to draw together, to diminish in bulk, and to prepare for fusion. A little after, it bubbled up and smoked. All the grains were united into one mass, without however forming a spherical button like other melted metals. After the platina had under gone this senlifusion, it was not attracted by the magnet as it was before the operation.
8. A portion of plating, deprived of the iron which it contained, and therefore not affected by the magnet, lost a part of its bulk, smoked, and formed one mass, which was extended under the hammer.• 9. Several experiments were made, in order to find the lens that was most proper for collecting the rays after refraction by the large lens. A spirit of wine lens 2 feet iu diameter and 4 feet focus, a solid lens 18 inches dia meter and 3 feet focus, and another 13 inches in diame ter, were successively tried, but none of them produced such a powerful effect as the lens 81 inches in diameter, and 22 inches and 8 lines focus, though it was full of vesi cles and strial.