248 Prof. E. Wiedemann on the 



former, being the difference of two curvatures, is itself a 

 curvature. If the curvature thus determined by shape 

 solely is expressed in dioptries, then, on multiplying by 

 the numeric which depends on the nature of the material, 

 the resultant power of the lens will also be expressed directly in 

 dioptries. The dioptrie spherometer facilitates this desirable 

 end by its directness of reading, quite apart from any 

 incidental advantages which may result from the details 

 of construction. 



XXXIV . On the Mechanics of Luminosity. 

 By E. Wiedemann. 

 [Continued from p. 163.] 



Experimental Measurements. 



17. TN order to show how many of the questions raised in 

 JL the foregoing admit of an experimental solution I have 

 made a number of measurements in various directions. The 

 method of investigation is briefly the following : — 



Since in all measurements the coefficients of emission are to 

 be expressed in calorimetric measure, i. e. in gramme-calories 

 per second, the numbers given by the amyl-acetate lamp, 

 serving as the source of light, for comparison at a given 

 distance from the photometer-slit must be reduced to absolute 

 intensity. This was done by comparison with the radiation 

 of .a glowing platinum wire. From these measurements we 

 obtain at once the energy in gramme-calories per second 

 radiated by 1 gr. platinum, and, consequently, relationships 

 between the total energy and that radiated in a definite region 

 of the spectrum, e.g. in the yellow. Next, the total bright- 

 ness of gas-flame coloured with sodium was compared with 

 the brightness of the amyl-acetate lamp in the yellow ; and 

 hence the coefficient of emission was calculated in gramme- 

 calories per second for one gramme and for a molecule of 

 sodium. 



Then follows the determination of the quantity 5, and from 

 its value that of the store of luminous energy L, and therefore 

 of the kinetic energy of the luminous motions. 



Dependence of the Radiation of a Platinum Wire upon its 



Temperature. 



18. The measurements were carried out in the first place 

 for the D-line. The amyl-acetate lamp was always employed 

 as the comparison-lamp. 



The arrangement followed in these measurements and for 



