164 INSTRUMENTATION 



amplitude ratio h falls in the range ^ ^ ^ 1.0. In a similar manner, 

 amplitude ratios in the range 1 ^ /i ^ go are obtained by rotating the 

 analyzer when the transmission direction of the polarizer is set parallel 

 to the direction of vibration of the fast ray in the complementary area. 

 If only the equivalent of A-type or B-type diffraction plates is wanted, 

 it is advantageous to cement the polarizer into the assembly of the 

 micoid disk. 



A great number of polanret systems are possible, and several systems 

 have already been suggested. At the present time, polanret systems 

 capable of varying both the amplitude ratio h and the phase difference d 

 can be successfully included in the standard microscope for objectives of 

 relatively long focal length and of low numerical aperture. Polanret 

 systems which vary the amplitude ratio at fixed values of the phase 

 difference present a less difficult optical problem and can be extended to 

 objectives of shorter focal length and of higher numerical aperture. 

 Multipupil systems afford an additional approach for introducing 

 polanret devices into the microscope. 



9.5. Application of Brewster's angle phenomenon lo variable 

 transmission phase microscopy 



If ordinary light is incident on a dielectric so that the angle of incidence 

 is equal to Brewster's angle, the light reflected by the dielectric is 

 linearly polarized. A fraction of only that component of the incident 

 light which is vibrating perpendicular to the plane of incidence is 

 reflected. Therefore, if linearly polarized light is incident on the 

 dielectric at Brewster's angle, then the amount of light reflected by the 

 dielectric can be varied by rotating the plane of vibration of the incident 

 light. No light is reflected if the incident polarized light vibrates 

 parallel to the plane of incidence, and the amount of reflected light is 

 maximum when the incident light vibrates perpendicular to the plane of 

 incidence. Frangon and Nomarski (1950a and 19506) have applied this 

 pi-inciple to vary the amount of light passing from the conjugate area 

 of the diffraction plate into the rest of the microscope system. Linearly 

 polarized light illuminates the specimen, and the undeviated light is 

 incident on the conjugate area of the diffraction plate at Brewster's 

 angle. Means are provided to rotate the plane of vibration of the light 

 falling on the diffraction plate (for example, by rotating the polarizer 

 placed in the light beam). The diffraction plate designed to take 

 advantage of this polarization phenomenon may reflect the light that 

 forms the phase contrast image into the remainder of the optical system 

 (Frangon and Nomarski, 19506), or it may be a transmission diffraction 

 plate incorporated into Frangon's hemispherical multipupil system for 



