280 ROSA AND CRITTENDEN: FLAME STANDARDS 



of melting were observed with an optical pyrometer. Hence, 

 from the true and the apparent temperatures of the oxide, the 

 emissivity can be computed. 



In the spectrophotometric method, a nickel strip was folded, 

 forming a wedge- or V-shaped cavity, and electrically heated 

 until a coat of the oxide was formed. It has been shown by 

 Mendenhall that the radiation from the interior of a V-shaped 

 cavity is '^black." The ratio of the intensities of the outside 

 and inside of the wedge, properly corrected for temperature 

 gradient through the walls, gives at once the emis^vity at any 

 particular wave length and temperature. Determinations by 

 this method agreed with those by the method of microscopic 

 melts. The monochromatic emissivity was found to increase 

 linearly with increasing wave length from X = 0.5 to 0.7^ and 

 decrease linearly with increasing temperature from 700 to 

 1300°C. For example, at 1160°C, Ex increases from 0.865 

 at 0.5m to 0.882 at 0.7)u; and for X = 0.65ju, Ex decreases from 

 0.958 at 800°C. to 0.845 at 1300°C. 



The total radiation of NiO was investigated by means of 

 twelve radiation pyrometers of the Thwing and Fery types. 

 Slighted upon the electrically heated oxide. The apparent 

 temperature of the oxide for X = 0.65/i was measured by an 

 optical pyrometer and these values were corrected to true tem- 

 peratures by means of the determinations on the monochromatic 

 emissivity. E was found to increase with increasing tempera- 

 ture but the relation is not linear. Temperatures and E have 

 respectively values as follows: 600°, 0.54; 800°, 0.68; 1000°, 

 0.76; 1200°, 0.85; 1300°, 0.87. Correction tables are required 

 for reducing the apparent temperatures of NiO, observed with 

 an optical or radiation pyrometer, to true temperatures. 



PHYSICS. — Flame standards in pliotometry. E. B. Rosa and 

 E. C. Crittenden. (To appear in full in the Bulletin of the 

 Bureau of Standards.) 



Although an agreement has been reached regarding the 

 relative values of the units of light in use in different coun- 

 tries, no one primary photometric standard has been generally 



