JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VII NOVEMBER 19, 1917 No. 19 



PHYSICS. — Criteria for gray radiation. Paul D. Foote, 

 Bureau of Standards. 



If certain materials, such as glowing carbon, are maintained at 

 a constant temperature and the brightness of these materials 

 compared spectrophotometrically with that of a black body at 

 various temperatures, it is well known that the logarithm of the 

 ratio of intensities for any wave length is a linear function of the 

 reciprocal of the absolute temperature of the black body and 

 that frequently the straight lines corresponding to various wave 

 lengths intersect in a single point. The writer and Mr. Fair- 

 child 1 have pointed out that this condition of the intersection of 

 the so-called logarithmic isochromatics is no criterion for gray- 

 ness of the non-black body. Dr. Hyde 2 has also established this 

 point , 



Dr. Nutting 3 in a recent paper concludes that the log isochro- 

 matics for a black body can not intersect in the manner de- 

 scribed, except over a limited range of wave lengths and that 

 the condition of approximate intersection and the condition for 

 grayness may both be satisfied, but either may hold without the 

 other 



We disagree with both of these conclusions as applied to our 

 work and desire to point out that the trouble is due partly to a 



1 Foote and Fairchild. J. Wash. Acad., 6: 194. 1916. 



2 Hyde. Ann. d. Phys., (4) 49: 144. 1916. 



3 Nutting. J. Wash. Acad., 6:476. 1916. 



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