GENERAL ACCOUNT OF RADIATION. 227 



the following results, the temperature of the radiating body being 

 120 



Lampblack . . . . . . 100 



Indian ink 

 Platinum 

 Copper 

 Silver 



, 88 



, 11 



5 



2 to 3 



They also found that the relative powers changed with the temperature. 

 For this purpose, they used a platinum plate coated on one side with 

 lampblack, and on the other side with the substance to be tested, and 

 heated it to various temperatures with an electric current. The radia- 

 tions on the two sides were compared by means of two thermopiles, one 

 placed on each side of the plate. Now, had the relative emissive powers 

 been independent of the temperature, the indications of the two thermo- 

 piles would have always been in the same ratio. But the ratio was found 

 to change. Thus the radiation of borate of lead was equal to that of 

 lampblack up to 100, but after that it fell in comparison, being only ^ at 

 550. This is exactly what we should expect, since the radiation is a 

 mixture of different wave-lengths or " colours," and we have no reason 

 to expect that the rise of temperature would produce the same increase in 

 emission of all the different wave-lengths. 



Radiation of Different Wave-Lengths. The difference of wave- 

 length in invisible radiation was shown conclusively by Forbes, who 

 succeeded in showing that as the temperature of a source rose, the 

 refractive index of the most energetic radiations emitted, as determined 

 by a rock-salt prism, rose also.* Langley,f using his bolometer, showed 

 that, while the refractive index of rock-salt for light ranges from about 

 T58 to 1'53, he was able to obtain radiation from a Leslie cube at 178 C., 

 which passed through a rock-salt prism with deviation corresponding to 

 a refractive index not greater than 1'45, and a wave-length probably 

 many times that of the D line. For the details of the work the reader 

 should consult the original papers, but a sketch of Langley's method may 

 serve to show how the quality of radiation may be investigated. 



When a beam of light from a slit falls on a diffraction grating, and 

 the rays are brought to a focus, a central bright band is formed, and a 

 series of spectra is arranged on each side, the distance of a given colour 

 in any spectrum from the centre being proportional to its wave-length. 

 The blue rays are therefore the nearest, and the red furthest, from the 

 centre in each. 



Considering the first spectrum on one side of the centre C, Fig. 134, 

 the various rays appear at points distant from the central bright band C 

 proportional to their own wave-length. If then Sj (Fig. 134) is the 

 position of the D line, and we erect a height S x D x equal to the wave- 

 length of the D line, 589/x/x,, and join OD a producing it onwards, the 

 height of this line above any point of CS 4 will represent the wave-length 

 of the ray of the first spectrum diffracted to that point. But even in the 

 solar spectrum there is no radiation perceptible below about 290/x/u,. 

 Hence, the spectrum only begins at A x where A X B X equals 290. But each 



* For an account of Forbes's researches on radiation ; see Balfour Stewart's Heat. 

 t Phil, Mag., xxi. 1886, p. 394 ; xxii. 1886, p. 149. 



