160 INSTRUMENTATION 



analyzer falls outside the quadrant between Ei and E2, as in Fig. III. 15B, 

 the projections OM and ON upon A are opposite in direction, and it may 

 be said that the directions of the electric vectors resulting from the 

 interaction of Ei and E2 with the analyzer are now anti-parallel. The 

 effect of rotating A out of the neutral quadrant between Ei and E2 is 

 therefore to produce an apparent discontinuous phase change of 3^ wave- 

 length between the unde\'iated and deviated waves which emerge from 

 the conjugate and complementary areas. If the construction of 

 Fig. III. 15 is repeated for opposite quadrants, it can be verified that 

 opposite quadrants are equivalent. Thus the neutral quadrants 

 are I and III, and the anti-neutral quadrants are II and IV. 



It is helpful to indicate the neutral quadrant on the scale of the 

 analyzer. The neutral quadrant can be determined from a knowledge 

 of the construction of the optical system, but it is usually easier to 

 establish the location of the neutral quadrant by any one of several 

 simple experimental procedures. First, the location of the four quad- 

 rants can be ascertained by removing the eyepiece and by rotating the 

 analyzer to the angle of extinction of the light emerging from the 

 conjugate area. Suppose, for example, that the observer has a slide 

 containing non-absorbing particles, the optical path of which exceeds 

 that of the surround by a small fraction of a wavelength. It is known 

 that such particles appear in bright contrast with a TA+0.25X diffrac- 

 tion plate and in dark contrast with a TA — 0.25\ diffraction plate. Let 

 the polanret system be of the type described in Section 9.2 and let 6 be 

 set at 4-45° so that 5 = 2^= -^90° or +X/4. When the microscope is 

 focused on the particles, these particles appear brightest when the 

 analyzer is rotated in the neutral quadrant and darkest when the 

 analyzer is rotated in the anti-neutral quadrant. As a check for 

 consistency, the experiment should be repeated with 6 selected at —45'^. 

 The particles should then appear brightest or darkest according as the 

 analyzer is rotated in the anti-neutral or neutral quadrants. The above 

 procedure is applicable e\^en when the quarter-wa\'e plate has been 

 omitted from the micoid disk and when a fixed phase difference of 

 5 = ±X/4 has been introduced between the conjugate and comple- 

 mentary areas by any one of the several possible methods. When the 

 phase difference between the conjugate and complementary areas has 

 been fixed at zero or 3^ wavelength, a suitable object specimen is a 

 silvered slide containing pinholes. If 5 = and if the objective is well 

 corrected for spherical aberration, the center of the Airy disk in the 

 sharply focused diffraction image of a small pinhole will appear brightest 

 or darkest according as the analyzer is rotated in the neutral or anti- 

 neutral quadrant. This situation is reversed when 8 = X/2. 



