M. E. Hagenbach's Experiments on Fluorescence, 57 



than the one above described, because they must furnish more 

 accurate results, provided that the form of the ring be verified 

 with equal exactness*. It has not yet been possible for me to 

 complete my work in the directions mentioned. 



The closer investigation of this subject might present many 

 and various points of interest. On the one hand, a deeper insight 

 would be thereby gained into the essence of that molecular pro- 

 cess which we designate as the magnetization of a substance. 

 From the facts here considered it seems to follow that the hypo- 

 thesis of the existence in iron of convertible molecular magnets, in 

 the form in which it is developed by Weber f, does not correspond 

 to the course of the phenomenon with feebler decomposing forces. 

 On the other hand, more exact knowledge of the function k might 

 be of practical utility, especially in the construction both of elec- 

 tromagnetic motors and of those magneto-electrical machines of 

 a newer kind (those of "Wilde, Siemens, Ladd, &c.) in which the 

 temporary magnetizing of iron plays so important a part. 



In conclusion, to Geh. Hofrath Kirchhoff, in whose laboratory 

 this research was carried on, and who kindly assisted me with 

 his advice, I most heartily express my gratitude. \ 



Heidelberg, October 18/1. 



VI. Experiments on Fluorescence. By Eduard HagenbachJ. 



IT had long been observed that a certain number of solutions 

 — for example, of nephritic wood (lignum nephriticum) — 

 possessed the property of giving, in incident light, a peculiar 

 reflection, quite different from the colour presented by the 

 same bodies in transmitted light. Brewster and Herschel were 

 the first to recognize that this property extended to a great 

 number of substances ; they thought to explain it, the one by 

 internal dispersion, the other by a peculiar reflection at the sur- 

 face. It is to Stokes that we are indebted for having established 

 that the change thus produced in the composition of* the incident 

 light does not result solely from reflection or absorption, but 

 from the substance itself becoming luminous under the influence 



* Equation (1) for example, is strictly correct only under the assumption 

 that in a cross section of the ring the number k can always be regarded as 

 a linear function of p. The Table of results proves that even with my ring 

 (in which the quantity p, or also the decomposing force, proportional to 



i, varies 10 per cent, within the cross section) this was almost everywhere 



P 



admissible. 



t Electrodyn. Maassbest. vol. iii. art. 26. 



% Abstract, by the Author, of the complete Memoir in Poggendoiff's 

 Annalen, vol. cxlvi. pp. 65, 232, 3/5, & 508. 



