14 STUDIES IN LUMINESCENCE. 



reference spectrum, as usual, was obtained from the light diffusely reflected 

 from a block of magnesium carbonate, but in this case the surface of the 

 block was illuminated by daylight, so that the fluctuations of intensity 

 might affect both spectra equally and in the same sense. 



The transmission of the solution was determined by placing a second 

 block of magnesium carbonate behind the cell and determining the intensity 

 of the light transmitted by the liquid in the various regions as compared 

 with that reflected from the first block. The instrument used in this and 

 in some of the subsequent measurements described in this chapter was a 

 spectroscope whose collimator was furnished with a double Vierordt slit. 

 Light for the comparison spectrum was introduced by means of a right- 

 angled prism and the adjustment of intensities was made in the usual 

 manner by means of micrometer screws. 



A second set of measurements with this substance was made with the 

 Lummer-Brodhun spectrophotometer. A mercury-arc lamp was used in 

 place of the Nernst filament of our previous experiments and the ultra- 

 violet line 0.365 ju, isolated by dispersion with the large spectrometer, was 

 employed for excitation. The comparison spectrum in this case was the 

 magnesium block illuminated by means of the acetylene flame. 



It is obvious that owing to the different distribution of intensities in the 

 spectra of daylight and of the acetylene flame these two sets of observations 

 would not be strictly comparable. In order, however, to make them at 

 least approximately so, the following correction was applied to the obser- 

 vations by daylight. From certain data obtained by Vogel 1 and published 

 by him many years ago, a curve was plotted giving the distribution of 

 intensities in the spectrum of diffuse daylight with clouded sky, the spectrun 

 of a petroleum-gas flame being taken as the standard. Vogel 's measure- 

 ments agree as well as could be expected, considering the fluctuating char- 

 acter of daylight, with those subsequently made by W. H. Pickering 2 and 

 by Nichols and Franklin. 3 As the sky was overcast at the time of making 

 the observations on quinine sulphate just described, it was assumed that 

 the quality of daylight would on that occasion be represented with sufficient 

 accuracy by means of this curve. By means of a similar curve, giving the 

 distribution of intensities in the spectrum of the acetylene flame as com- 

 pared with the petroleum flame, it is possible to determine the relation 

 between daylight and the acetylene flame. Vogel's curve for daylight- 

 petroleum and the computed curve for daylight-acetylene are given 4 in Fig. 13. 

 By means of the latter it is possible, from the observations upon quinine 

 sulphate, to compute a curve in which the light of the acetylene flame 

 reflected from magnesium carbonate is the standard. This curve (B, Fig. 

 14) corresponds precisely, as regards the location of its maximum, with 

 curve A in the same figure, which shows the results of the measurement of 

 the fluorescence of this substance when excited by means of the ultra-violet 

 of the mercury lamp. Since these two sets were made with different types 

 of spectrophotometer their agreement affords a desirable check upon the 

 performance of the two instruments. 



Vogel, Berliner Monatsberichte, p. 801, 1880. 



2 W. H. Pickering, Proc. Am. Acad, of Arts and Sciences, vol. 25, 1880. 

 3 Nichols and Franklin, American Journal of Science, vol. 38, p. 100, 1889. 



The two sources being so adjusted as to have the same intensity at the D line, the curve shows the ratio 

 of intensities for other wave-lengths. 



