140 



BURNING INSTRUMENTS. 



ON DIOPTRIC BURKING INSTRUMENTS. 



Dioptric 

 burning 

 instm- 

 meats. 



Tschirn- 

 hausen't 

 burning 

 leu?. 



Account 

 of iu effects 

 upon differ- 

 ent bodies. 



There is reason to believe that the ancients were 

 acquainted with the use of burning lenses, though 

 the information upon this point which has been con- 

 veyed to our times is extremely trifling. All that we 

 know on the subject, indeed, is contained in Pliny 

 and Laetantiiis. The former of these authors men- 

 tions globes of glass and crystal, which, when exposed 

 to the su-n, burnt the clothes and flesh on people's 

 backs ; and Laetantius observes, 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 

 constructed by M. 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 in- 

 crease the force of the solar rays, the light refracted 

 by the large lens was received upon a lens of a smal- 

 ler size, which converged them to a point nearer the 

 large lens than its principal focus, and had n focal im- 

 age of only 8 lines in diameter. The large lens, which 

 weighed 160 pounds, was purchased by the Duke of 

 Orleans, and presented by him to the French Acade- 

 my. As its effects were extremely powerful, we shall 

 give them at some length. 



1. 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 pro- 

 per size, were easily melted. 



4. Tiles, slates, delft ware, pumice stone, talc, 

 whatever 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 

 porcelain. 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 sub- 

 stances 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 sud- 

 denly into cold water, produced such a violent con- 

 cussion 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 

 transparent 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; becsme opaque some days after. 



16. Substances which the heat renders at first 

 transparent, but which afterwards become opaque by !nstn 

 being melted with other substances thn 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. 



M. De BufFon, whose burning mirrors have al- rj u flr 01 

 ready been noticed, directed his attention also to the burning 

 construction of burning lenses. His first object was lenses with 

 to form burning glasses, by combining two circular water, 

 segments of a glass sphere, so as to form a lenticular 

 cavity to be filled with water. These glass segment^ 

 wore first moulded into their proper shape, and then 

 regularly ground on both sides, so that the concrve 

 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 seg- 

 ments 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 

 the rays sustained, produced a very great degree of 

 aberration. BufFon also proposed to make each seg- 

 ment consist of a number of smaller segments put to- 

 gether into a frame ; but as the water could not ear 

 sily be prevented from insinuating 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. 



In order to avoid the great thickness of glass at Buffon's 

 the centres of large convex lenses, BufFon has pro- burning 

 posed a very ingenious contrivance. Instead of ma- sw _' . 

 king the burning lena of one piece of glass, he pro- 

 poses to form it of three concentric circular pieces 

 resting upon each other. Thus if the whole diame- 

 ter of the lens is to be 24 inches, which would require 

 a central thickness of three inches if it were of so- 

 lid glass, the middle part will be a lens 8 inches in 

 diameter, with a thickness of 1 inch. This lens 

 is inserted in the middle of a circular zone, whose di- 

 ameters 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 

 surfaces of the lens and of the two zones are 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 diameter. The 

 great advantages which are gained by this construc- 

 tion, 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 diminu- 

 tion of thickness, the power of theltns 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 be 

 transmitted through the lens of 8 inches, and will be 

 twice as powerful as if they had been refracted by a 

 similar portion of a solid and continuous lens. Trudaine's 



The next burning lens of any magnitude was con-. burnii><* 

 structed by M. Bernieres for M. Trudaine de Mon- lens. 



