ON DIOPTRIC BURNING INSTRUMENTS.
There is reason to believe that the ancients were ac quainted with the use of burning lenses, though the in formation upon this point which has been conveyed to our times is extremely trifling. All that we know on the subject, indeed, is contained in Pliny and Lactantius. The former of these authors mentions globes of glass and crystal, which, when exposed to the sun, burnt the clothes and flesh on people's backs ; and Lactantius ob serves, that a sphere of glass filled with water, and held in the sun, lighted the fire, even in the coldest weather.
The first burning lenses of any magnitude were COD trUCt.Cd by bl. Tschirnhausen. They were three and four feet in diameter, their focal length was about twelve feet, and the diameter of the focal image about an inch and a half. In order, however, to increase the force of the solar rays, the light refracted by the large lens was received upon a lens of a smaller size, which converged them to a point nearer the large lens than its principal focus, and had a focal image of only 8 lines in diameter. The large lens, which weighed 160 pounds, was pur chased by the Duke of Orleans, and presented by him to the French Academy. As its effects were extremely powerful, we shall give them at some length.
I. All sorts of wood, whether hard or green, and even when wet, were burnt in an instant.
2. Water in a small vessel boiled immediately.
3. All the metals, when the pieces were of a proper size, were easily melted.
4. Tiles, slates, delft ware, pumice stone, talc, what ever was their size, grew red and vitrified.
5. Sulphur, pitch, and resins, melted under water.
6. When the metals were placed in charcoal, they melted more readily, and were completely dissipated.
7. The ashes of wood, vegetables, paper, and cloth, were converted into a transparent glass.
8. All the metals were vitrified upon a plate of porce lain. Gold received a fine purple colour.
9. Substances that would not melt in pieces were easily melted in powder ; and those that resisted the heat in this form, melted by adding a little salt.
10. A substance easily fused assists in melting more refractory substances when placed along with them in the focus ; and it is very singular, that two substances which are very difficult to melt separately, are very easily melted when exposed together, such as flint and English chalk.
11. A piece of melted copper being thrown suddenly into cold water, produced such a violent concussion that the strongest earthen vessels were broken to pieces, and the copper was thrown off in such small particles that not a grain of it could be found. This did not happen with
any other metal.
12. All bodies except the metals lose their colour. The precious stones are instantly deprived of it.
13. Certain bodies vitrify easily, and become as trans parent as crystal ; but by cooling they grow as white as milk, and lose all their transparency.
14. Other bodies, that are opaque when melted, be come beautifully transparent when they are cooled.
15. Substances that are transparent, both when melted and cold, become opaque some days after.
16. Substances which the heat renders at first trans parent, but which afterwards become opaque by being melted with other substances that are always opaque, produce a beautiful glass, always transparent.
17. The rays of the moon concentrated by this lens, though extremely brilliant, have no heat.
Dc Buffon, whose burning mirrors have already been noticed, directed his attention also to the construc tion of burning lenses. His first object was to form burning glasses, by combining two circular segments of a glass sphere, so as to form a lenticular cavity to be fill ed with water. These glass segments were first mould ed into their proper shape, and then regularly ground on both sides, so that the concave and convex surfaces were exactly parallel. The one which he constructed was 37 inches in diameter, with a focal length of about 5 feet and a half, and the segments were of considerable thickness, to prevent. them from breaking or altering their form by the weight of the included water. As the refractive power of water is very small, Buffon proposed to increase it by saturating it with salt ; but notwithstanding every precaution, he found that the focus of lenses of this kind was never well terminated, nor reduced to its smallest size, and that the different refractions which thc rays sus tained, produced a very great degree of aberration. Buf fon also proposed to make each segment consist of a number of smaller segments put together into a frame ; hut as the water could not easily he prevented from in sinuating itself between the joints of the segments, and as there would be a great difficulty in arranging them in the same spherical circumference, this kind of burning glass does not seem to have ever been executed.