258 Prof. E. Wiedemann on the 



radiating surface is 



4 4 \ej 



If a square centimetre emits a quantity of energy E', then 

 our surface yields a quantity of energy, 



irrh 



Of this the fraction which reaches the unit length of the 

 slit is s/^ttt)' 2 . We must here divide by the whole surface of 

 the cone, since the sodium flame is transparent to its own 

 rays. The quantity of energy actually falling upon the slit is 

 therefore 



The distance rf thus does not occur in the final result, since 

 the radiating surfaces increase as the squares of the distances. 

 We may say that the quantity A' is the fraction of the total 

 energy which passes through the diaphragm. Strictly speak- 

 ing, account should also be taken of the circumstance that the 

 flame represents not a space bounded by two parallel very 

 large surfaces, but a cylinder. Nevertheless, what we thus 

 neglect is small in comparison with the other sources of error. 

 We have further neglected the fact that the slit is not a 

 portion of the sphere, but occupies a tangent plane. 



(c) We therefore obtain for the ratio of the energies 

 which reach the slit from an extended source of light, and a 

 narrow linear source 



Af__ 1 /A\2mgE' K-ia(i\ S A ' 



A~16\e) 8 E ' 0r E ~ lb \h) 2tt V A ' 

 With the dimensions of our apparatus in particular 



E' A 7 A' 



J- = 0'24^, or E'=0-24|-E. 



The ratio of the energy of a source of light with a con- 

 tinuous spectrum, and that of the platinum wire at a definite 

 point of the spectrum is obtained at once from the readings 

 of the photometer. We have seen above that the brightness 

 of the platinum is 1*827 times greater than that of the amyl- 

 acetate lamp for the yellow in the neighbourhood of the D 

 line. Hence A/A' =1-827, 



A'/A = 0-547, 

 and we obtain for the energy of unit surface of the amyl- 



