58 M. E. Hagcnbach/s Experiments on Fluorescence. 



of incident light and emitting radiations whose refrangibility is 

 different from that of the exciting rays. It was, again, this 

 savant who introduced into science the word "fluorescence" to 

 designate this phenomenon, on account of its being particularly 

 noticeable on certain varieties of fluor spar. 



Stokes studied a great number of substances with respect to 

 their fluorescence, and in this way arrived at the important law 

 that the refrangibility of the light of fluorescence is never 

 greater than that of the exciting rays. The results obtained by 

 Stokes have since been confirmed and extended by the labours 

 of other physicists, particularly M. V. Pierre. 



I was induced to undertake in my turn the study of this sub- 

 ject, because from different quarters there were sent to me, to 

 be submitted to a more searching examination, substances pos- 

 sessing in a high degree this interesting property. I soon per- 

 ceived that a theory of the singular phenomenon would not be 

 possible until we possessed more exact observations than those 

 which had hitherto been made. I extended my researches not 

 only to these new substances, but also to those previously 

 studied. I turned my attention more particularly to three 

 points, namely : — - 



1st. Determination of the limits and the maxima of fluores- 

 cence : the question here was, to ascertain in what parts of the 

 spectrum fluorescence begins and ends — and to see in each par- 

 ticular case if there was one or several maxima, always fixing 

 their exact position. 



The method which I made use of for this purpose consisted 

 in the direct projection of the solar spectrum on the surface of 

 the liquid. 



2nd. Study of the absorption-spectrum of the fluorescent 

 substances, and verification of the relation, first clearly esta- 

 blished by Stokes, between absorption and fluorescence — consist- 

 ing in this, that in all portions of the spectrum where there is 

 fluorescence there is also absorption of the exciting rays. 



3rd. Spectral analysis of the light emitted by fluorescence. 

 Here the object was, to know the limits between which the spec- 

 trum produced by fluorescence extends, and to fix the position 

 of the maximum or maxima, according as the spectrum presented 

 only one or several luminous bands separated by dark portions. 

 In this part of my w^ork I applied myself to find out especially 

 whether modifications in the composition of the incident light 

 (that is to say, that which excites the fluorescence) involve mo- 

 difications in the light emitted by the fluorescent substance : 

 with this aim I employed different sources of light for its pro- 

 duction. 



The method employed in these researches was, to throw the 



