REFLECTION OF RADIANT HEAT. 353 



in suitable positions, all the luminous rays may thus he cut 

 off, and only invisible ones obtained. In this way any required 

 portion of either the least refrangible or the most refrangible 

 rays may be separated from the visible ones, so that any kind 

 of invisible ray of particular refrangibility may be obtained as 

 required. 



For the most refrangible ultra-violet rays, absorption by a 

 deep blue transparent medium, such as ammoniacal solution 

 of copper, may also be employed ; all visible light of lower 

 refrangibility (red, orange, yellow, and green rays) being absorbed, 

 and only the indigo, violet, and ultra-violet rays being allowed to 

 pass. 



Expt. 378. Reflection of Radiant Heat Burning Mirror. By 

 the aid of a flat mirror and thermometer it may readily be shown, as 

 above described, that radiant heat is reflected along the same path 

 as the visible rays emanating from the same source. Accordingly 

 a concave mirror will collect to a focus a beam of radiant heat, 

 just as it will visible light. A burning mirror is simply a concave 

 reflecting surface ; if the curvature be spherical, and such a mirror 

 be turned directly towards the sun, the heat rays will be reflected 

 along with the visible ones, and will all become concentrated 

 to a focus at or near a point midway between the surface of 

 the mirror and its centre of curvature on the prolongation of 

 the line joining the sun's centre and that centre of curvature. 

 If the mirror be large enough, inflammable objects placed at 

 this point (paper, gunpowder, &c.) will be fired, so that a cigar 

 may be thus lit ; even if it be small, the concentration of the 

 heat rays is still readily perceptible on placing the bulb of a ther- 

 mometer at this geometrical focus, or the head of a match, or a 

 little bit of phosphorus.* 



Expt. 379. Two Conjugate Mirrors. Obtain two similar con- 

 cave reflectors about 10 or 12 inches diameter, and with a radius 

 of curvature of some 12 or 15 inches; the workmanship need not 

 be very fine, circular sheets of copper roughly hammered into shape 

 answering the purpose sufficiently well when the surface is tinned 

 or silvered, or even rubbed over with quicksilver, and polished so 

 as to reflect light pretty brightly. Arrange the two mirrors exactly 



* Tradition states that Archimedes set fire to the Roman fleet at Syracuse 

 by means of large concave mirrors. Button succeeded in firing a tarred 

 wooden plank at 70 yards' distance by what was equivalent to a lar^e concave 

 mirror, viz., a muuber of comparatively small plane mirrors (128, each about 

 8 inches long and 5 broad), arranged close together and so inclined to the 

 path of the sunlight that each one reflected the incident light to the same 

 spot, 70 yards off. 



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