228 



HEAT. 



ray is also diffracted into the other spectra ; in the second, at double the 

 distance from C ; in the third at treble the distance from 0, and so on. 

 Hence if we make CS 2 = 2 CSj ; CS 3 = 3 CS 15 &c., the D line of the second 

 spectrum is at S 2 , of the third at S 3 , &c. Making S 2 D 2 = S 3 D 3 = SjDp and 

 joining CD 2 , CD 3 , &c., the heights of these lines above any point give us 

 the wave-lengths of the various spectra diffracted to that point. Drawing 

 through Bj a parallel to CjSj, we need not consider any line below that, 

 as the corresponding radiation is insensible. 



It is evident that, except near the centre, the spectra overlap. At 

 the point S 4 , for instance, we have 



A =589 of the 4th 



= | x 589 of the 3rd 



= 2 x 589 of the 2nd 



= 4 x 589 of the 1st. 



The first of these is, of course, the yellow line easily seen, the second is 

 in the far red, the others dark rays. 



If, now, the slit of a spectroscope be placed at S 4 , and a rock-salt train 



FIG. 134. 



of lenses and prisms be used to minimise absorption, since the different 

 rays have different refractive indices, they are sorted out into different 

 positions in the field of the spectroscope. The bolometer is made to 

 travel along, first being placed in the yellow line, which is visible. As 

 it passes each of the others in succession, the effect is apparent, and 

 though the ultra-red rays are invisible the bolometer feels them. By 

 this method, Langley was able to detect radiation received from bodies 

 at quite low temperatures, and in positions in the spectrum correspond- 

 ing to very great wave-lengths. 



Comparison of Absorptive Powers. The absorptive powers of 



various substances were first compared by Leslie, who covered his 

 thermometer bulb with a layer of lampblack, and placed it in the focus 

 of the mirror, noting the rise, and then compared this with the rise 

 when the bulb was covered with other substances. This method is not 

 satisfactory, as the thermometer is steady only when the absorption is 

 equal to the emission, and the emission as well as the absorption varies 

 with the substance. Melloni put plates of thin copper between the 



