1879.] on the Eleciric Light. 21 



solved in a proper vehicle iodine cuts tlio visible radiation sharidy 

 oft', but allows the invisible free transmission. We have hitherto 

 used it dissolved in bisulphide of carbon. By fusing together iodine 

 and sul})hur, Professor Dewar has recently added to the number of 

 our eftectual ray-filters. It may be made as black as pitch for the 

 visible, while remaining transparent for the invisible rays. By such 

 filters it is possible to detach the invisible rays from the total 

 radiation, and to watch their augmentation as the temperature rises. 

 Expressing the radiation from a platinum wire when it first feels 

 warm to the touch — when, therefore, all its rays are invisible — by 

 the number 1, the invisible radiation from the same wire raised to a 

 white heat might be 500 or more. An actual table of measurements 

 will clearly show the gradual growth of the invisible radiation as a 

 spiral of platinum wire rises from darkness to an intense white heat. 



Dark 1 



Dark, but hotter 3 



Dark, but still hotter 5 



Dark, but still hotter 10 



Feeble red 19 



Dull red 25 



Eed 37 



Full red 62 



Orange 89 



Brigbt oraugo 144 



Yellow 202 



White 27(> 



Intense white 440 



It is not then by the diminution or transformation of the non- 

 luminous emission that we obtain the luminous ; the heat rays maintain 

 their ground as the necessary antecedents and companions of the light 

 rays. When detached and concentrated these powerful heat rays can 

 produce all the effects ascribed to the mirrors of Archimedes at the 

 siege of Syracuse. While incompetent to produce the faintest glimmer 

 of light, or to affect the most delicate air-thermometer, they will inflame 

 paper, burn up wood, and even ignite combustible metals. When they 

 impinge upon a metal refractory enough to bear their shock without 

 fusion, they can raise it to a heat so white and luminous as to yield, when 

 analyzed, all the colours of the spectrum. In this way the dark rays 

 emitted by the incandescent carbons are converted into light rays of 

 all colours. Still, so powerless are these invisible rays to excite vision 

 that the eye has been placed at a focus competent to raise platinum 

 foil to bright redness without experiencing any visual, or even thermal, 

 impression. Light for light, no doubt, the amount of heat imparted by 

 the incandescent carbons to the air is far less than that imparted by gas 

 flames. It is less because of the smaller size of the carbons, and of the 

 comparative smallness of the quantity of fuel consumed in a given 

 time. It is also less because the air cannot penetrate to the interior of 



