4 INTRODUCTION TO PHASE MICROSCOPY 



crease the contrast of the specimen; he also actually reversed it, causing 

 the light strips to appear darker than the coated ones. This was done 

 by replacing the specimen strips of soot with thin strips of platinum. 

 Bratuscheck pointed out that by substituting platinum for soot he 

 slightly changed the phase of the light which had passed through the 

 platinum lines with respect to the phase of the light passing through the 

 open spaces between. These observations were made before 1892 at the 

 time when Abbe was much concerned with establishing the diffraction 

 theory of image formation in the microscope, and they were aimed to 

 substantiate such knowledge. It is certain that they did not find their 

 way into practical applications of the microscope. 



In outlining the earlier experiments and theoretical considerations 

 which form a background for the subject of phase microscopy the work 

 of A. E. Conrady and J. Rheinberg should be included. In 1905 

 Conrady published experimental results showing that a phase reversal 

 exists in different orders of diffraction spectra produced by the light 

 diffracted by specimens in the form of gratings. As will be shown later, 

 this is an important fact in phase microscopy. Rheinberg (1905) used 

 a grating in which the slits were twice as wide as the bars. He screened 

 off all the spectral orders except the first- and second-order spectra on 

 one side and showed a complete reversal of contrast, providing an 

 example of contrast control in the image by gaining control of phase in 

 the diffraction spectra through proper selection of these spectra. 



It is to Professor F. Zernike, of the University of Groningen, that we 

 are indebted for the first application of phase contrast principles to the 

 microscope and for an explanation of these principles. Zernike's first 

 published work (1934) on this subject showed the advantage of the phase 

 contrast method over the familiar knife-edge method in testing the 

 quality of optical systems. In the same year, C. R. Burch (193-4) 

 published his results on the experimental application of the then-new 

 Zernike method to the evaluation of defects in concave reflecting mirrors. 

 This original application and its later important incorporation in micro- 

 scopes appear as a logical step when we consider that errors in optical 

 systems and concave mirrors are detrimental, because they impress 

 unwanted differences in the paths of various portions of the transmitted 

 or reflected light wave, and that in the testing of such systems it is es- 

 sential to observe and evaluate these path differences. In a somewhat 

 similar manner, in phase microscopy the structures within the specimen 

 impress path and, consefjuently, phase differences that we wish to detect 

 visually and localize within the specimen. 



In 1935 Zernike discussed the application of the phase contrast method 

 to microscopy, although patent rights granted to the firm of Carl Zeiss 



