VISIBLE AND NEAR-VISIBLE RADIATION 



141 



cools. The intensity of radiation during the period of freezing, usually 

 lasting from 5 to. 15 min., has been found to be reproducible within the 

 experimental error of observation, i.e., less than 0.25 per cent. For 

 further details, the reader should consult the original papers (62). 



This type of source has been made the standard of both brightness and 

 color temperatures. For a discussion of its application to color tempera- 

 ture problems, the reader is referred to the publication of the National 

 Bureau of Standards (65). A brief discussion of its application to bright- 

 ness is not only of value in itself, but will serve to illustrate the inter- 

 relation between the photometric or illuminating system and the absolute 

 system. 



Since Jx is the power emitted per square centimeter per unit solid 

 angle in a direction normal to the surface, if one multiplies by the abso- 

 lute visibility, one obtains the brightness per unit wave-length. If, 

 now, this value is integrated over the entire wa^'e-length range, one 

 obtains the total brightness of the source. As has been noted before, 

 the absolute visibility is obtained from the relative visibility by multi- 

 plying by the maximum absolute visibility, or the reciprocal of the least 

 mechanical equivalent of light. The expression for brightness becomes, 

 therefore 



B 



1 



0.001602 



I 



JexV^dX 



the constant factor having been taken outside the integral sign. In the 

 application of this formula it should be noted that usage differs as to the 

 quantity /. Our values apply to the power emitted in unit solid angle, 

 whereas the values given in the original article refer to the total power 

 emitted in the hemisphere. Where the latter is the case, tt appears in the 

 denominator of the constant factor. 



Table 8 



* Palladium point, t Platinum point, t Rhodium point. § Iridium point. 



