264 



PROGRESS IN MICROSCOPY 



According to Stokes's law, the emitted light is of longer wave- 

 length than the exciting light. When excited by the ultra-violet 

 radiation, the fluorescent elements of the specimen emit visible ra- 

 diation. Hence, all the optical elements beyond the specimen P, the 

 microscope and the spectrograph, may be of glass. 



Fig. 10.20. Arrangement for fluorescence microspectrophotometry. 



The layout may be the one shown in Fig. 10.20. The source 5 

 emits the ultra-violet radiation. By means of the optical system L, S 

 is imaged in the condenser C which is adjusted so as to image the 

 diaphragm D on the specimen (Kohler illumination). The filter K 

 set for instance, close to Z), occludes the visible light, letting through 

 only the exciting ultra-violet radiation. The optical elements between S 

 and the specimen P should absorb the least possible ultra-violet as, 

 for a given exciting radiation and given observation conditions, the 

 intensity of the fluorescence-emitted light is proportional to the ex- 

 citing-light intensity. Uviol glass, quartz or reflecting devices are 

 suitable. 



As in the layout shown in Fig. 10.11, the specimen F is imaged 

 on the slit F of the spectrograph by the microscope whose objective 



