GENERAL CONSIDERATIONS OF DESIGN 79 



altered by a change in the thickness of the cover glass. Since spherical 

 aberration affects the performance of the phase objective, it is often 

 important to select cover glasses having a thickness deviating by not 

 more than 0.02 or 0.03 mm from the thickness for which the phase 

 objective has been adjusted. 



The angular spread of the light deviated by diffraction at the object 

 specimen depends on the dimensions and shape of the specimen and on 

 the optical properties of the specimen relative to those of its surround. 

 Some specimens deviate the light in such a manner that the amount of 

 deviated light which passes through the conjugate area is negligible. 

 With other specimens this will not be true if the size and location of the 

 conjugate area remain the same. When optimum contrast is demanded 

 in the image, a change in the optical properties of the specimen relative 

 to those of the surround i-equires a change in the relative amplitude 

 transmission and in the optical path difference between the conjugate and 

 complementary areas of the diffraction plate, even when the deviated 

 light is completely separated from the undeviated light. The changes in 

 the amplitude transmission and optical path of the diffraction plate are 

 quantitatively different when the overlap of the deviated and un- 

 deviated light at the conjugate area becomes sufficiently great. The 

 favored amount of contrast depends largely on the specific application 

 of the phase microscope. It is not necessary to produce greatest possible 

 contrast in order to study the detail in a specimen; in fact, gradations in 

 contrast may offer additional information. The amount of contrast 

 preferred even for a given specimen may be different according as visual 

 observations or photographic records are made. 



There exists a large group of object specimens which differ from their 

 surround by a small amount in optical path but which do not differ 

 appreciably from their surround in absorption. The optical path 

 difference is said to be small when it is less than 3^ wavelength. Be- 

 cause the useful range of contrast is wide, it is possible to design a 

 diffraction plate that is satisfactory for observing this entire group of 

 specimens. The size and location of the conjugate area of such a 

 diffraction plate can be determined by the two rules of thumb mentioned 

 previously in this section. The energy transmission of the conjugate 

 area is chosen to fall between 25% and 40% of the energy transmission 

 of the complementary area. The magnitude of the optical path differ- 

 ence between the conjugate and complementary areas is made equal to 

 y^ wavelength at 5401 A. Suppose that the optical path through 

 the conjugate area exceeds the optical path through the complementary 

 area by 3^ wavelength. Then a particle will appear in bright or dark 

 contrast according as its optical path is greater than or less than that of 



