while the other passes through an ultraviolet microscope to another 

 photoelectric cell. By interposing the sample in the beam passing through 

 the microscope, the reduction in intensity of the light passing through the 

 sample may be determined by comparison of the photoelectric current 

 generated by the two beams. This gives the ratio /, //„ directly without 

 moving the sample and reduces the exposure time of the sample to ultra- 

 violet. The per cent transmission (T) of the specimen is determined 

 from the ratio h/h, which can then be used to compute the amount of 

 ultraviolet absorbing substance in arbitrary units. This method differs 

 from that used to measure the absorption of Feulgen-stained nuclei in 

 the visible spectrum only in the use of ultraviolet radiation as the light 

 source and a microscope equipped with optics transparent to the shorter 

 ultraviolet wavelengths. While more elaborate ultraviolet microspectro- 

 photometric techniques have been developed, the above will serve to 

 introduce the student to the fundamentals of ultraviolet absorption 

 measurements. 



Polarization Microscopy 



The polarizing microscope is similar in principle and construction to 

 the conventional optical microscope. It differs only by the addition of 

 polarizing elements in the optical system to permit illumination of the 

 specimen with plane polarized light instead of ordinary white light. The 

 two chief polarizing elements are the polarizer and analyzer (Figure 

 1 1-22) . The polarizer, usually a sheet of Polaroid film, is mounted below 

 the substage condenser and admits linear-polarized light vibrating in a 

 single plane into the optical system of the microscope. The analyzer is 

 rotatable, and is mounted above the objective lens in the body tube of 

 the microscope. When the analyzer is rotated through 360°, the field of 

 view appears alternately bright and dark for every 180° turn, that is, 

 for every complete rotation of the analyzer there are two positions at 

 which maximal transmission of light occurs, and two positions at which 

 no transmission occurs. Maximal light transmission by the analyzer re- 

 sults when its axis of transmission is parallel to that of the polarizer 

 (Figure 11-22). No transmission ("extinction") of light occurs when 

 the axis of the analyzer is set at right angles to that of the polarizer 

 (Figure 11-22). At extinction, the polarizer and analyzer are spoken of 

 as being in the "crossed" position. By placing an object between crossed 

 polarizer and analyzer any effect the object has on the natural path of 

 the polarized light will become immediately apparent. 



242 / CHAPTER 11 



