PHENOMENA OF PHASE MICROSCOPES 



25 



complementary areas are equal. We have seen that in the ordinary 

 microscope the deviated 1) wave, Fig. II. -4, is retarded by practically 

 3^ wavelength with respect to the imdeviated S wa\'c when the optical 

 path of the particle exceeds the optical path of the surround by a small 



^Condenser 

 diaphragm 



Fig. II. 5. Passage of the light radiated from a point C in the opening of the con- 

 denser diaphragm through the optical system of a phase microscope. The undeviated 

 portion of the light wave diverging from any point C in the opening of the condenser 

 diaphragm is converged upon the conjugate area of the diffraction plate located near 

 the plane of the image of the condenser diaphragm and near the second focal plane 

 of the objective. The conjugate area coincides with the image of the opening of the 

 condenser diaphragm. All the undeviated rays from points C pass through the 

 conjugate area in a properly designed and adjusted system. The deviated rays are 

 distinguished by the broken lines. They arise by diffraction at the particle and may 

 occupy the entire shaded cross section and the entire area of the diffraction plate. 

 If the conjugate area is small compared with the total area of the diffraction plate, 

 most of the deviated rays pass thi'ough the complementary area, i.e., through that 

 area of the diffraction plate which is unoccupied by the image of the opening of the 



condenser diaphragm. 



amount, A, for a particle whose amplitude transmission is substantially 

 the same as that of the svuTound. When A is very small, the amplitude 

 of the D wave is less than that of the S wave and becomes vanishingly 

 small as A approaches zero. 



Suppose that the undeviated S wa^'e is artificially retarded by J^ 

 wavelength. This artificial retardation can be brought about by apply- 

 ing a coating of refracting material, such as magnesium fluoride, to the 

 conjugate area of the diffraction plate. If the thickness of the magnesium 

 fluoride is such as to increase the optical path of the conjugate area by 

 }4: wa\'elength with respect to the uncoated complementary area, only 



