Fluorescence of Dyes on Ware-Length of Ex citing Light. ol\S 



The voltage can be regulated by means of a rheostat K. 

 A trial has shown. that the change of voltage from 100 to 

 110 does not influence perceptibly the relation of inten- 

 sities measured by the spectrophotometer. The vessel F by 

 means of slides not represented in the figure can be moved 

 with a micrometrical screw along the screen. In such a 

 way d can be regulated. The same installation served for 

 measurement of the absorption spectra of the dyes studied, 

 light-filters, and for determining the distribution of energy 

 of the exciting light. For the measurement of absorption of 

 light-filters, the widths of the slits of the spectro-photometer 

 were taken as O'l-O^. mm. (measurements of intensity of 

 fluorescence were made with open slits, i. e. the spectro-photo- 

 meter served rather as a photometer). Instead of vessel F a 

 prism of total reflexion was put and the aperture d 1 was closed 

 by a ground glass. Thus the light passing light-filter A 

 enters the spectro-photometer. The second measurement was 

 made without a light-filter. Hence the coefficient of absorp- 

 tion was determined as usual. The absorption of solutions 

 of dyes was measured in different ways : (1) by means of a 

 vessel with Schultz's body; (2) by an immediate installation 

 of an absorption vessel in the passage of the parallel light- 

 bundle betwesn l 6 and S x ; (3) for determining the slopes 

 of the curves of absorption the measurements were made in 

 vessels of considerable thickness (6-8 cm.). 



The Distribution of Energy. 



A Hefner's candle was used as a standard. Its distri- 

 bution' of" en erg'y in the visible spectrum has been carefully 

 measured by Angstrom *, and can be expressed quite precisely 

 through the formula of a grey radiation : 



7-85 



I A = 0'0160.\- 5 .<r~^, .... (9) 



(X in microns). The formula was frequently tested. The 

 measurements of Nichols & Merritt and Ooblentzf show 

 thaj the perceptible deviations begin only in the red region 

 of the spectrum, approximately from 605 /jl/j, : i. e., behind the 

 limits of interval studied by us. We can evaluate the 

 exactness of (9) according to the difference of the integral 

 radiation of the Hefner's candle, measured frequently, and 



* K. Angstrom, Phys. Rev., I. xvii. p. 302 (1903). 

 t Cf. C. L. Nichols and E. Merritt, ' Studies in Luminescence, 1 

 p. 178 (Washington, 1912). 



