OPTICAL FUNDAMENTALS 5 



indicate that Zernike realized the importance of the phase contrast 

 method in microscopy as early or earlier than 1932. However, it was 

 not until 1941 that Kohler and Loos, of the firm of Carl Zeiss, published 

 a paper showing the results of their extensive and convincing experiments 

 with the Zernike method. The pul)lication of this paper stimulated much 

 interest in the method, especially among those concerned with the design 

 of microscopes. Burch and Stock (1942) described their experiments 

 with a slit-shaped light source and an accelerating Zernike strip in the 

 back focal plane of the microscope objective. This system had the merit 

 of ease of construction, but manufacturers have adopted the ring form 

 of substage annulus and Zernike strip because, as pointed out by Kohler 

 and Loos, this form of phase plate with its complete axial symmetry 

 introduces practically no undesirable lack of symmetry into the 

 image. 



Although the possibility is mentioned by Kohler and Loos, it is interest- 

 ing to note that the earlier experiments with phase contrast microscopy 

 show no indications of the use of absorbing material in the Zernike strip 

 until Zernike (1942) published photomicrographs, at least one of which 

 was taken with a strongly absorbing phase strip. 



The image produced by a microscope cannot duplicate the object 

 specimen completely. This inability arises largely from the finite wave- 

 length of light (or other electromagnetic radiation) and the limited 

 numerical aperture of the optical system. Hence, the microscopist may 

 encounter optical images that may, unless properly interpreted, give 

 incorrect information regarding the size, shape, and uniformity of micro- 

 scopical specimens. When a new method of microscopy is under trial, 

 the question of interpretation of the image becomes important. Phase 

 microscopy offered no exception to the requirement of interpretation 

 (Anon., 1934). The phase method reached its present wide acceptance 

 only after careful theoretical and experimental studies of the correct 

 interpretation had been made. The theoretical considerations and ex- 

 perimental results presented in subseciuent chapters indicate that 

 images produced by phase microscopy are not more difhcuJt to interpret 

 than those produced by other methods of microscopy. In practice, by 

 means of a series of phase microscope objectives, different views of the 

 specimen can be obtained, each adding to the composite information 

 obtainable regarding the specimen. 



3. OPTICAL FUNDAMENTALS 



This chapter will not present in detail the theory on which phase 

 microscopy is based. In later chapters and in the Appendix may be 

 found a detailed mathematical presentation of this subject. However, 



