116 



PROGRESS IN MICROSCOPY 



of the compensated interference eyepiece. The optical elements to the 

 right of P in said figure are identical with those in Fig. 3.24. The 

 additional element brought in is the Savart polariscope, placed in 

 any position ahead of the condenser C. The on-infinity fringes of the 

 polariscope are beamed in the condenser focal-plane F^, plane which 

 is always ahead of the specimen P. 



The objective O^ images the plane F^ at Fq,^ and, owing to the 

 short focal length of O^ , the plane Fq,^ is not far from the objective's 

 focal plane. The fringes of the polariscope ^i are seen at F^,,,. Provided 



% 



m 



'y. 



Fjg. 3.25. Principle of tiie compensated interference eyepiece. 



the focal length of the lens O., is suitable, the plane Fq,^ can be made 

 to match the focal plane of the eyepiece lens O.. whereupon the S^ 

 fringes and the S.y virtual fringes are in the same plane. Provided the 

 spacing of the fringes, originated from S^ and So, be the same and 

 the two polariscopes in the proper direction, then birefringences 

 cancel out. 



There is no longer need to diaphragm the condenser which may 

 be used with its standard diaphragm. 



As in all polarizing devices, the polarizer 2*i is ahead of S^ and the 

 analyser SPo next after S.. 



Let us change the objective Oi\ if the focal length is different, 

 so is the position and spacing of the fringes of ^i. Theoretically, 

 the polariscope S^ should be changed, too. In fact, the solution is 

 much simpler: merely setting the condenser nearer to or farther from 

 the specimen will serve the purpose. This motion alters the magni- 

 fication between the planes F^- and F^, and, hence, the fringe spacing 

 shown by S^ at iv,., . Compensation is well correlated. In the latter 

 adjustment, the Kohler illumination is diverged from owing to the 

 condenser motion. To obviate this difficulty, the mechanical member 

 securing S^ to the condenser comprises a small revolving plate fitted 

 with two divergent lenses and a free aperture. Rotating the plate 



