BEOREATIVK SCIENCE. 



and outtiiclo the focus, its inverted image is Men fringing in tho 

 air. 



-11 diverging rayu proceeding from the hand 

 made converging whon they fall upon tho mirror 06, ainl 

 verted image ia seen at tn, tho diverging raja e d being brought 

 to a point at o t, whcro they cross and enter tho eye. 



iu this manner that the concave mirror haa been 

 nsed, probably from tho moat 

 anoiont times, to produce 

 illusory speotros, and by con- 

 cealing tin- mirror, and illu- 

 minating tho object properly, 

 very amusing effects may be 

 shown. Nearly every old work 

 on optics gives a diagram 

 and directions for using the 

 concave mirror, of which Mr. 

 Walker's is probably ono of 

 tho best. 



In Fig. 12 let a be tho 

 mirror, d the actor concealed 

 by tho cross partition, c, e a 



light, also concealed by 

 the partition t. If d holds a 

 book, or any other object, the 



male* use of flat mirron for burning. Ration Mya the astoaishi&f 

 pbilosophico-miliUry exploit of Archimedes ha* been recorded 

 by Diodorus Hioulu*. Lucian, Dion, Zonaras, Oaten, Anthemio*, 

 Tsetses, and other ancient writera. The aocoont of TieUes U M 

 particular that it suggested to Father Kireber the specific 

 method by which Archimedes effected his purpose. " Arthi. 

 medc t " aays that author, " set fire to the fleet of MaroeUiu by 



a bnrninjf-gbus, composed of 

 mall square mirron, moving 

 every way on hinges; and 

 which, when placed in the son's 

 ray, reflected them on to the 

 Boman fleet, so M to rednee it 

 to ashes at the distance of a 

 bow-shot." This account gained 

 additional probability by the 

 effect which Zonarae ascribes 

 to the burning mirror of Pro- 

 clos, by which he affirm* that 

 the fleet of Vitellins, when be- 

 sieging Byzantium, now Con- 

 stantinople, was utterly oon- 

 Knmed. But perhaps no histo- 

 rical testimony could havo 

 gained belief to such extraor- 



Fig. 8. 



Pig. 10. 



light reflected from it will pass 

 between the screens, or parti- 

 tions, c and i, to the mirror, and 

 be from thence reflected to z, 

 where tho image of the book 

 will appear so tangible, that 

 the spectator, looking through 

 the opening x, will suppose ho 

 could take hold of it. The con- 

 federate, d, may actuate vari- 

 ous moving figures, as flying 

 birds, angels, demons, etc., tho 

 effects of which at a would bo 

 Very surprising. 



A concave mirror becomes a 

 burning mirror when held in the 

 sun. The heat at the focus is 

 very powerful. It is not, how- 

 ever, supposed that this was the form of reflector used by 

 Archimedes when he destroyed tho fleet of Marcollus by con- 

 centrating tho rays of the sun upon his ships. The celebrated 

 Kirchcr went to Syracuse, and observed that tho Roman ships 

 could not have been at a less distance from the walls of the 

 city than thirty- three paces ; and, as the focus of a concave 

 mirror is at the distance of half the radius, he calculated that the 

 concave mirror used by Archimedes must have been a portion of 

 a sphere of at least forty-six paces radius, and therefore most 

 difficult, if not (in those days) impossible to construct. Yitellio 

 states that Anthemius of Tralles, the engineer who lived in tho 

 time of the Emperor Justinian, was the first rho proposed to 



Rf.9, 



dinary facts, if similar one* 

 had not been seen in modern 

 tunes. In the memoirs of tho 

 French Academy of Sciences 

 for 1726, p. 172, we read of a 

 plane mirror of twelve inches 

 square reflecting the son's rays 

 to a concave mirror sixteen 

 inches in diameter, in the focus 

 of which bodies were burnt at 

 the distance of 600 paces. The 

 great naturalist Buffon con- 

 structed a burning apparatus 

 by combining 168 pieces of 

 looking-glass six inches by 

 eight, HO that he could, by me- 

 chanism connected with each, 

 concentrate the rays of the 



sun to one focus ; and by using 224 mirrors he was able to melt 

 plates of silver at a distance of forty feet ; with 112 mirrors he 

 set fire to planks covered with wool at a distance of 138 feet; 

 oven with twelve mirrors combustibles could be inflamed at a 

 distance of twenty feet. 



It was with the concave mirror the ancients re-kindled the 

 sacred fire, so carefully watched by the vestal virgins. PlnUrch, 

 in his life of Nnma Pompilius, says that the instruments used 

 for this purpose were dishes, which were placed opposite to the 

 sun, and the combustible matter placed in the centre by which 

 it is probable he meant the focus, conceiving that to be at tho 

 centre of the mirror's concavity. 



