STUDIES IN LUMINESCENCE. 



which, especially with reference to the validity of Stokes's law, the long- 

 continued discussion already described arose. But few attempts have 

 been made to apply the spectrophotometer to the study of fluorescence; 

 yet it is obviously possible to determine both the limits and the maximum 

 of a spectral region for which a curve of intensities can be plotted with 

 far greater accuracy than by the method hitherto pursued by all observers, 

 i. e., that of attempting to set the cross-hair in the eye-piece of a spectro- 

 scope in the region of greatest brightness, or at the point where the spectrum 

 ceases to be visible. Experimenters have perhaps been deterred from the 

 use of the spectrophotometer because of the faintnessof fluorescence spectra. 

 It is true that the fluorescent light from many substances is so weak as to 

 preclude all measurements of its spectrum; but it is also true, as we have 

 found in the course of the experiments to be described, that settings can 

 be made in cases where the brightness of the spectrum is far below that 

 necessary to arouse the sense of color and where the presence of light can 

 be detected only after prolonged shielding of the eye. The use of the 



Fig. i. 



cross-hair in such cases is out of the question, for the field is much too dim 

 to render it visible, while every attempt to illuminate it from the side would 

 flood the eye-piece with light sufficient to quench that under observation. 



The instrument used in most of our observations was the spectropho- 

 tometer of Lummer and Brodhun. In many of our earlier measurements 

 the ocular lenses in the eye-piece were used, the eye being focused upon the 

 aperture in the eye-piece and not upon the face of the prism. By means of 

 metal screens attached to the collimator slits of the instrument the length of 

 slit was regulated so as to avoid overlapping of the spectral images and to 

 give two contiguous spectra in the field of view. One loses in this way the 

 advantage of the method of contrast, but the brightness of the field is 

 greatly increased. Later on the contrast field was employed because it was 

 found to be less fatiguing to the eye and on the whole more accurate. Since 

 in most cases it was desired to employ monochromatic light for the excita- 

 tion of fluorescence, the spectrophotometer was employed in connection 

 with a large spectrometer, as shown in Fig. i . The eye-piece and slit of the 



