MICROSPECTROSCOPY AND MICROSPECTROPHOTOMETRY 



265 



is Oi. Under the incident ultra-violet radiation, some elements of the 

 objective may become fluorescent. Such fluorescence brings about an 

 undesirable haze which must be occluded by a filter within the objec- 

 tive, which occludes the ultra-violet radiation. 



The problem then arising is the following: having obtained the 

 fluorescence spectrum emitted by a detail of the specimen, it is now 

 desired to compare it with that of another detail, either of the selfsame 

 or of another specimen. 



The same procedure as used previously achieves this as follows: 

 a portion of the spectrograph slit F is covered by the fluorescent image 

 and the remainder by the image of a comparison source exhibiting 

 a continuous spectrum. A rotatable sector is set on the latter portion 

 which shows, on the photographic plate, a series of continuous spectra 

 correlated to various illuminations whose relative values are thus 

 determined. Exposure time is the same for either the fluorescence 

 spectrum or the comparison spectra. Then, at every wave-length, the 

 curve connecting the measured densities (d) (photographic densities 

 measured on the comparison spectra) to the logarithms of relative 



d, (ioqE) 



Fig. 10.21. Curve connecting (d) (photographic density) to the absorbance d. 



illumination can be plotted. Relative-illumination logarithms may 

 be substituted for densities d, which may define the illumination of the 

 plate. The higher the density, the greater the illumination received. 

 Let us now consider a fluorescent detail whose measured density 

 is (di). This is the density of the fluorescence spectrum at the wave- 

 length A. The curve connecting (d) to ^exhibits a density rt'ilFig. 10.21). 

 Such density defines the intensity of the fluorescence emitted by the 

 detail at the wave-length /. Changing the wave-length enables to plot 

 the curve d = f{?^), defining the fluorescence change of the detail 

 versus the wave-length. 



