114 INSTRUMENTATION 



exceeding that of the surround by a small fraction of a wavelength. 

 This is the type of contrast observed in ordinary brightfield microscopy 

 with such a specimen, although the contrast is very much less. The 

 phase objectives for dark contrast which are supplied by a given maker 

 include a series of focal lengths. Achromatic objectives for dark 

 contrast with focal lengths of approximately 16 mm and 8 mm, initial 

 magnifications of approximately 10 X and 20 X, and numerical apertures 

 of approximately 0.25 and 0.50, respectively, are made by the American 

 Optical Co., Bausch and Lomb Optical Co., R. and J. Beck, Ltd. (1949), 

 Cooke, Troughton and Simms, Ltd. (1949), C. Reichert Co. (1949), 

 Henry Wild Surveying Instruments Supply Co., Ltd. (1949), and Carl 

 Zeiss, Jena (1949). A dark-contrast achromatic objective with a focal 

 length of approximately 4 mm, initial magnification of approximately 

 43 X, and a numerical aperture of about 0.65 is available from American 

 Optical Co., Bausch and Lomb Optical Co., Cooke, Troughton and 

 Simms, Ltd., Reichert Co., Wild Co., and Zeiss, Jena. A dark-contrast 

 objective of 4-mm focal length but with N.A. 0.85 is offered by Beck, 

 Ltd. R. Winkel G.M.B.H. (Zeiss-Winkel, 1949) lists two dry achro- 

 matic objectives for dark contrast: lOX, N.A. 0.25; and 42 X, N.A. 0.85. 

 Officine Galileo (1949) offers for dark contrast three dry achromatic 

 objectives: 10.5 X, N.A. 0.28; 25 X, N.A. 0.65; and 52 X, N.A. 0.88. 

 The series of dry, achromatic objectives for dark contrast available from 

 L Koristka and Co. (1949) are described as 12 X, N.A. 0.30; 26 X, N.A. 

 0.65; 45 X, N.A. 0.85; and 60 X, N.A. 0.87. Immersion phase objec- 

 tives which produce dark contrast are supplied by all manufacturers of 

 phase microscopes, and the manufacturers and phase objectives may be 

 summarily listed as follows: American Optical Co., 1.8 mm, 97 X, oil, 

 N.A. 1.25, achromatic; Bausch and Lomb Optical Co., 1.8 mm, 97 X, 

 oil, N.A. 1.25, achromatic; Beck, Ltd., 2 mm, oil, N.A. 1.30, achromatic; 

 Cooke, Troughton and Simms, Ltd., 1.8 mm, 95 X, oil, N.A. 1.30, 

 achromatic; Officine Galileo, 3'i2 inch, 100 X, oil, N.A. 1.30, achromatic; 

 I. Koristka and Co., M inch, 55 X, oil, N.A. 0.90, achromatic; Koristka, 

 ^2 inch, 100 X, oil, N.A. 1.30, achromatic; Koristka, Ks inch, 100 X, 

 water, N.A. 1.20, achromatic; Koristka, Ks inch, 100 X, oil, N.A. 1.32, 

 semi-apochromatic; Reichert Co., 100 X, oil, N.A. 1.25, achromatic; 

 Wild Co., 2.2 mm, 85X, oil, N.A. 1.25, achromatic; Zeiss-Winkel, 90X, 

 oil, N.A. 1.30, achromatic; and Zeiss, Jena, 90X, oil, N.A. 1.25, achro- 

 matic. The series of achromatic phase objectives for dark contrast 

 made by the American Optical Co. includes both type TA— 0.25X and 

 type TB— 0.25X, and an objective of a given focal length is supplied 

 with one of several values of T in either type. The different values of T 

 produce different amounts of contrast with a given object specimen. 



