10 COBLENTZ: CONSTANTS OF SPECTRAL RADIATION 



sufficiently good definition and magnification, and a large enough 

 field of view for melting point observations. The working dis- 

 tance of the objective may be increased, if desired, by the device 

 employed by Robin^ of inserting a biconcave lens at D slightly 

 back of the focus of the objective. This necessitates lengthening 

 considerably the draw tube and reduces the area sighted upon. 



The micropyrometer may, of course, be used for the estimation 

 of temperatures of incandescent surfaces simultaneously with 

 their examination, and thus becomes a useful instrument in metal- 

 lographic and microchemical and physical investigations at high 

 temperatures. 



The apparatus is being used at the Bureau of Standards for 

 the determination of melting points and emissivities of the re- 

 fractory elements and alloys, and the author will be glad' to 

 receive pure samples (a few hundredths milligram), particularly 

 of the rarer elements for such determinations. 



PHYSICS. — The constants of spectral radiation of a uniformly 

 heated enclosure or so-called black body. W. W. Coblentz, 

 To appear in The Bulletin of the Bureau of Standards. 



In previous communications (Physical Review, 1910 and 1911) 

 accounts were given of the progress made in the investigation of 

 the constants involved in the formulas, ' proposed by Wien and 

 by Planck, for expressing the partition of energy in the spectrum 

 of a so-called black body. 



In view of the fact that the great mass of theoretical specula- 

 tions seemed entirely out of proportion to the slender experi- 

 mental data upon which they were based, it seemed desirable 

 to obtain an extensive series of observations under all sorts of 

 conditions, leaving the exact computation of the results until the 

 very last, and then computing all the data on a uniform basis. 

 From the beginning of the work, four years ago, attention was 

 called to the fact that the Wien equation does not fit the 

 observed spectral energy curves, and in later communications, 

 the computations were made on the basis of the Planck equa- 



* F. Robin, Microscope k longue portee, etc. Bull, de la Soci^te d'Encourage- 

 ment 118 : 204. 1912. 



