252 Prof. E. Wiedemann on the 



Within the visible spectrum the like is shown by older 

 experiments. Thus, M. Yiolle finds that between 954° and 

 1775° the intensity for the C-line increases 154 times, for the 

 D-line 219 times, and for the E-line 307 times. The inten- 

 sity therefore increases the more slowly the less refrangible 

 the rays are. Moreover the total radiation contains the infra- 

 red rays, whose increase will be much slower than is shown 

 by the above numbers. At the same time we see that even 

 at the highest temperature the radiation in the infra-red 

 furnishes an overwhelming contribution to the total radiation. 

 For the extraordinarily rapid increase of the radiation in the 

 visible part of the spectrum is not sufficient, as the numbers 

 given above show, to compensate the very much slower 

 increase in the infra-red. 



Herr H, Schneebeli* has obtained the same results in expe- 

 riments with Swan lamps. Whilst the total radiation increased 

 in the proportion of 1 to 4, the optical radiation increased in 

 the proportion of 0*3 to 24. The Swan lamps are, however, 

 scarcely so highly exhausted but that one must suppose that 

 a portion of the loss of energy is due to convection. Further 

 H. Schneebeli employed a gramme-machine to furnish the 

 current, of which the current-strength varied between certain 

 maximum and minimum values. The quantity of energy 

 given off in the unit time is then, if we denote by J the mean 

 current-strength as shown by the galvanometer, and by i that 

 existing at any time t, and by w the resistance regarded as 

 constant, not u-J 2 , but f 1 wi 2 dt. It is not possible to decide 



between these two values without knowing what the action of 

 the Gramme-machine was. 



When Hr. E. Lecher f nevertheless, in a research on the 

 visible spectrum, is able to make the brightness equal in all 

 parts of the spectra yielded by two different platinum wires 

 by weakening the whole of the brighter of the two spectra, 

 the reason no doubt is that, as the above-mentioned experi- 

 ments of M. Violle show, the changes in the visible spectrum 

 for the different colours are not remarkably different. More- 

 over, for the small brightnesses in question, only quite limited 

 regions of the spectrum contribute to the physiological light- 

 sensation % . 



* Wied. Ann. xxii. p. 433 (1882). f Ibid. xvii. p. 512 (1882). 



% I wish to avail myself of this opportunity to make a remark on the 

 construction of formulae which represent the emission as a function of the 

 temperature and the wave-length. With the exception of the formula of 

 Herr Stefan, according to which the radiation is proportional to the fourth 

 power of the absolute temperature, all have more or less the character of 

 interpolation-formulae. 



If a formula, based upon theoretical considerations, is really to represent 



