A SPECTROPHOTOMETRIC STUDY OF FLUORESCENCE- II 



fluorescence spectrum, to produce fluorescence of sufficient strength for 

 measurement with the spectrophotometer, it follows that observable fluo- 

 rescence can be produced by light of even greater wave-length than that 

 recorded in our diagram. 



We deem the evidence already given in the foregoing paragraphs to be 

 conclusive, so far as these substances are concerned; but in view of the 

 differences of opinion among physicists as regards the validity of Stokes's 

 law we venture to add the following description of a determination of the 

 wave-length of the least refrangible monochromatic light which we found 

 capable of exciting fluorescence in the three solutions with which this 

 chapter deals. 



To insure freedom from the existence of possible errors due to stray 

 light, two different methods were employed of avoiding it. In the first 

 the exciting light, before dispersion, was passed through a solution of the 

 substance to be examined, thus filtering out those rays particularly active 

 in producing fluorescence. The filtered light was then dispersed by means 

 of the large spectroscope already described, and a second solution was 

 subjected to an isolated, nearly monochromatic, region of the spectrum. 

 The wave-length of this region was increased until the last trace of fluo- 

 rescence, observed directly with the eye, 



was about to disappear. To distinguish Ta ble i. 



between fluorescence and the presence 

 of light diffused from small particles, 

 the light was viewed at an angle of qo 

 through a Nicol prism, by which means 

 diffuse light was completely excluded. 

 The limit of excitation thus determined 

 lay, as had been anticipated, farther to 

 the red than in the cases where a spectro- 

 photometrically measurable fluorescence 



had been obtained, excepting in the case of eosin, where it was found to 

 coincide almost exactly with the ultra edge of the band used in exciting 

 the spectrum shown in curve A (Fig. 9). 



The second method consisted in sending the isolated region of the 

 dispersed light to be used for excitation through a second spectroscope and 

 determining as before the limit of excitability. This method of removing 

 stray light has been extensively used and is well known to be effectual. 

 The source of light in both cases was an electric arc. 



The results obtained by these two methods were identical and are shown 

 in Table 1. 



These wave-lengths, all of which lie far to the red from the maximum 

 of the fluorescence spectrum, are indicated in Figs. 3, 8, and 9 by means 

 of the vertical lines marked /. 



The results obtained with solutions of fluorescein, eosin, and naphthalin- 

 roth thus fully confirm the contention of Lommel 1 that in the case of these 

 substances Stokes's law is not fulfilled. In the opinion of Lommel all other 

 fluorescent substances, with the probable exception of chlorophyll, conform 

 to Stokes's law, the shortest wave-length of the fluorescence spectrum being 



'Lommel, Poggendorff's Ann., 143, 159, and 160. 



