260 Prof. E. Wiedemann on the 



In our case 



a = 0*0 2 83. 



If we make the slit so wide that when illuminated by homo- 

 geneous light of wave-length X it has a breadth in the spectrum 

 corresponding to a difference of wave-length A at this place, 

 and if we now illuminate it with white light, then every point 

 at the same place receives rays between the wave-lengths X 

 and X + A. 



If the ordinate corresponding to the wave-length X in the 

 energy-curve is y, and that corresponding to X + A is y l9 then, 

 since A is always small, the area included by the ordinates y 

 and y Xj the curve, and the axis of abscissa is 



and the corresponding energy is 



The breadth A of the slit illuminated by the sodium flame 

 in our experiments amounted to 0*22 of the distance between 

 the sodium and lithium lines in the spectrum ; the wave- 

 length of the sodium line is 0*59, of the lithium line 0*67. 

 Each point of the spectral image receives then rays between 

 the wave-lengths X = 0*59 and X + A = 0*59 + (0'67-0*59) 

 0-22 = 0-6076. Further, to the abscissae 0'59 and 0*6076 cor- 

 respond the ordinates y — 11*35 and y l = 13*33 ; the above- 

 mentioned surface is therefore 



F= ll-35 + 13-33 x0 . 0176= 24^ x0 . om 



But to this surface there corresponds a fraction f of the 

 total energy 



f=0*0 2 83x ^P- x 0*0176 = 0*00180= -t-. 

 2 556 



Having thus determined the energy corresponding to this 

 definite breadth of slit from measurements with our apparatus, 

 we find for the sodium flame the whole, but for the platinum 

 wire only the ^Iq of the total radiated energy. 



A' and A are the measured brightnesses of the sodium 

 flame and of the platinum wire, in reference to that of the 

 amyl-acetate lamp ; they are proportional to the squares of the 

 cotangents of the readings on the photometer. 



A' = const. cotan 2 a', A = const. cotan 2 a, 



where the constants have the same value. 



