POLARIZING MICROSCOPE 



(a) (b) (C) 



Fig. 1. Appearance of the back aperture of a 97 X 1.25 NA strain-free coated objective with and 

 without rectifiers. The condenser, which is identical with the objective, is used at full aperture. Colli- 

 mated mercury green light (546 ni/j) used for illumination, (a) Crossed polarizers, no rectifier, (b) Po- 

 larizer turned 2°, no rectifier, (c) Crossed polarizers, with rectifiers in both condenser and objective. 

 Photographs a, b, and c were given identical exposures. From Inoue, S. and W. L. Hyde (12). 



lenses has long been recognized and polariz- 

 ing microscopes are generally furnished with 

 "strain-free" lenses. The magnitude of 

 strain BR in these lenses is frequently still 

 considerable and for realizing the utmost 

 sensitivity, lenses with exceptionally low 

 strain BR must be selected. The procedure 

 for selecting these follows. 



Preparation for the Test. Provide a polariz- 

 ing microscope with a very high intensity 

 light source, such as a high pressure mercury 

 arc lamp (General Electric A-H6 or Osram 

 HBO 200) or other lamp of equivalent 

 brightness. The components of the micro- 

 scope should be adjusted or repaired so that 

 the EF of the system without the lenses is 

 greater than 10^. Close down the field stop 

 and use Kohler illumination, employing a 

 nominally identical objective as the con- 

 denser. It is convenient to screw the test 

 lens into an objective holder which in turn 

 is placed inverted on the rotating stage and 

 centered to the optical axis. This acts as the 

 condenser (or objective in an inverted sys- 

 tem as in Fig. 2) and the selected mate re- 

 mains fixed. In addition prepare a removable 

 rotating mica compensator of the order of 

 X/20 to be placed in the slot above the ob- 

 jective or between the condenser and the 

 polarizer. 



Test for Freedom from Strain BR. When 

 the pair of lenses is aligned, the polars crossed 



and the image of the field stop properly 

 focused a dark cross (Fig. la) should appear 

 at the back aperture of the objective. In the 

 absence of strain BR the cross is observable 

 with objectives of NA as low as 0.1 wath no 

 object lying between the condenser and ob- 

 jective lenses. The cross must be symmetrical 

 and very dark between crossed polars and 

 should open symmetrically into two hyper- 

 bola-hke fringes the "V's" (Fig. lb) when 

 the polarizer or analyzer is rotated. The arms 

 of the Vs should remain dark until they dis- 

 appear beyond the edge of the aperture. If 

 the lenses are free from local and lateral 

 strain these dark fringes remain undistorted 

 when the stage is rotated and the test lens is 

 examined at various orientations. 



Some lenses seemingly passing the above 

 test may still suffer from radially symmetric 

 (strain) BR. This is checked by inserting the 

 mica compensator. When the lenses are 

 free from radial as well as lateral BR the cross 

 loses contrast and fades away with little 

 change in its shape or position when the 

 compensator is rotated. If lenses possess ra- 

 dial BR the cross will open into two V's as 

 though with a strain-free lens the analyzer 

 had been turned. 



Rectifiers. Lenses selected for freedom 

 from strain by the methods described can 

 give extremely high EF at low NA's (e.g., 

 3 X 10^ at condenser NA = 0.1). However, 



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