CHAPTER III 



INSTRUMENTATION 



1. GENERAL CONSIDEKATIONS OF DESIGN 



The preceding chapters have shown that the ordinary microscope can 

 be converted to a phase microscope by the addition of two essential 

 elements, namely, the diffraction (or phase) plate and a specialized con- 

 denser diaphragm. Thus the schematic diagram of Fig. II. 5 contains a 

 condenser diaphragm with an annular opening as aperture located at the 

 first focal plane of the substage condenser and a diffraction plate located 

 at the second focal plane of the objective. The condenser diaphragm 

 and the diffraction plate perform definite functions in the phase micro- 

 scope. The performance of these functions is modified by both the 

 specimen and the optical design of various components of the microscope. 

 The spherical aberration of the objective and curvature of field in the 

 image of the condenser diaphragm, for example, may introduce limita- 

 tions. The minimization of the adverse effects produced by these 

 modifications leads to some of the most difficult problems of designing 

 and making a phase microscope. 



It has also been shown in the previous chapters that the main fvmction 

 of the restricted opening in the condenser diaphragm is to provide an 

 optical means for separating the light that is deviated by diffraction at 

 the object specimen from the light that remains undeviated by diffraction 

 at the object specimen. This separation occurs at the diffraction plate. 

 The undeviated rays obey the laws of geometrical optics and pass 

 through that area of the diffraction plate which is occupied by the 

 geometrical image of the opening in the condenser diaphragm. This area 

 is called the conjugate area. The remaining area of the diffraction plate 

 intercepts most of the deviated rays and is called the complementary 

 area. The chief function of the diffraction plate is to alter the amplitude 

 ratio between the undeviated and deviated waves and to change the 

 phase difference between these two waves. To accomplish this the 

 amplitude transmission and the optical path of the conjugate area are 

 deliberately made different from the amplitude transmission and the 

 optical path of the complementary area by any one of several methods. 



The essential requirement governing the relative location of the con- 

 denser diaphragm and of the diffraction plate is that the condenser 



75 



