224 HEAT. 



not in the external disturbance, but in the effect upon which we 

 concentrate our attention. 



Radiant Energy has a much greater Range of Wave-length 



than Light. But though we now believe that all light is radiant energy, 

 the converse is obviously not true, for we may receive heat by radiation 

 from an absolutely dark body in a dark room. To illustrate the distinc- 

 tion let us consider the formation of a spectrum. If a beam of sunlight, 

 or the light from an electric lamp falls on a slit S (Fig. 132), and passing 

 thence is received on a lens L, it will.be brought to a focus at a point 

 F, the focus conjugate to S, and an image of the slit is formed on a screen 

 placed at F. If a prism P be interposed at the angle of minimum devi- 

 ation, then the light is bent round, each colour at a different angle, and 

 forming its own image, so that a spectrum consisting of an infinite num- 

 ber of overlapping slits of different colours is formed on a screen rb, 

 placed at the same distance from the prism as F, the red being the least, 

 the violet the most refracted. In various experiments on interference 

 of light we obtain a similar spectrum, arranged in the same order from 

 red to violet. Now, whatever view we hold as to the nature of the dis- 



FIG. 132. Radiation spread out into Spectrum. 



turbance constituting light, there can no longer be any doubt that it is a 

 wave-form of energy, this supposition affording the only conceivable 

 explanation of interference phenomena. And it can be shown that 

 difference in position in the interference-spectrum must correspond to 

 difference in the length of wave constituting the light. We learn thence 

 that red light has the longest wave, the length diminishing through the 

 spectrum to the violet. The total range of wave-length of the visible 

 spectrum appears to lie between ^ ^^ and Tr A^ of an inch, or between 

 000075 cm. and '00004 cm. 



We may turn the light into heat by placing a thermopile at various 

 parts of the spectrum. At the red end it is sensibly affected, but less 

 and less as it moves from that end, the heating being quite insensible to 

 ordinary experimental arrangements in the blue. It was formerly 

 supposed that the visible spectrum was accompanied by a heat spectrum 

 especially strong in the red. 



Now, were there any difference in kind between heat radiation and 

 light, we might expect by some process to filter out the one from the 

 other. But we cannot separate red radiation into two parts, one warm- 

 ing and the other lighting. When we diminish the light, we equally 

 diminish the heat, as Jamin has shown.* We conclude, then, that there 

 is a radiation of one kind only for each wave-length. The eye trans- 

 lates the radiation of the different wave-lengths into different colours ; 



* Cours de Phytique : Etude des Radiation, p. 62. 



