14 WRIGHT: AN IMPROVED VERTICAL-ILLUMINATOR 



constant as observed in the 9 sets of observations, aggregating 

 90 spectral energy curves which were obtained in 1912. While 

 it is very gratifying to find that the new value of co = 14,500 

 places the melting point of platinum within 1° or 2° of the ob- 

 served values (which are uncertain by a similar amount), this 

 coincidence is to be considered somewhat accidental. 



Planck's theory of the mechanism involved in the production 

 of "black" radiation admits of the computation of the value of 

 the elementary electrical charge, e, by means of the constant of 

 spectral radiation, h, (XmT = 2920) and the constant, a, of total 

 radiation. The recent determinations of the constant of total 

 radiation, by Gerlach and by Puccianti give a value of a- = 5.9 X 

 10~^2 watts per cm.^ per deg.^ Combining this with the value of 

 h = 0.2920 cm. deg. gives a value of e = 5.1 X 10"" E. S. U. 

 This is considerably higher than the experimental value by Milli- 

 kan, which is 4.777 X lO-^^E. S. U. While the time is still pre- 

 mature to attempt to harmonize the experimental value and the 

 theoretical value as deduced from the radiation laws, it is of 

 importance to notice that there are wide variations in the various 

 determinations of the constant, a, of total radiation. In future 

 communications it is hoped to contribute values of a, which are 

 determined with some of the apparatus used in determining c?. 



Heretofore, the experimental work on the spectral energy 

 curves was done in the winter time when the humidity was low. 

 The season of low humidity being close at hand, it is purposed 

 to observe several more series of energy curves, using other fluor- 

 ite prisms, before publishing a more complete report. As matters 

 now stand it seems highly desirable to put the complete optical 

 path of the spectroradiometric apparatus and of the radiator in 

 a vacuum in order to obtain the complete spectral energy curves 

 free from the atmospheric absorption bands of water vapor, 

 carbon dioxide, and oxygen. 



METALLOGRAPHY. — An improved vertical-illuminator. Fred 

 Eugene Wright, Geophysical Laboratory. 



Of the two types of vertical-illuminators which are in current 

 use on the metallographic microscope, the reflecting prism type 

 furnishes the best illumination with low power objectives. With 



