SUBSTAGE CONDENSERS AND CONDENSER MOUNTS 115 



At present, fewer manufacturers supply phase objectives that form a 

 bright-contrast image of a particle ha\'ing an optical path greater than 

 that of the surround by a small fraction of a wavelength. I. Koristka 

 and Co. offers as extensive a series of objectives for producing bright 

 contrast as for producing dark contrast. The firm of Galileo lists an 

 achromatic oil immersion oV^jective, ,/i2 inch, 100 X, and N.A. 1.30, 

 for producing bright contrast. The American Optical Co. makes 

 bright-contrast achromatic objectives as follows: IG mm, 10 X, N.A. 

 0.25; 8 mm, 20 X, N.A. 0.50; 4 mm, 43 X, N.A. 0.66; and 1.8 mm, 

 97 X, oil immersion, N.A. 1.25; and several objectives of a given focal 

 length can be obtained to produce different amounts of bright contrast. 

 The American Optical Co. also supplies a phase objective in. which the 

 coating that forms the diffraction plate consists of only an absorbing 

 film applied over the conjugate area (TA+SX, < 5 ^ 0.05). 



The phase objectives mentioned above have an annular conjugate 

 area. C. Baker, Ltd. (1950), has been making phase objectives in 

 which the conjugate area is cruciform. These phase objectives produce 

 dark contrast and include the standard three dry achromatic objectives 

 and an achromatic oil immersion objective. 



4.2. Substage condensers an<l condenser mounts 



The substage mount carries the condenser and the condenser dia- 

 phragm. When each diaphragm is inserted separately beneath the 

 condenser and must be remox'ed from the substage unit before another 

 diaphragm can be used, the condenser mount is designated as the single- 

 unit type. Such a single-unit mount is available from the American 

 Optical Co. for use with an Abbe-type condenser, N.A. 1.25. Each 

 diaphragm is contained in a cell which is screwed into place below the 

 condenser whenever the diaphragm is needed. Figures III. 5. la and 

 III. 5. 16 show, respectively, a cell containing a diaphragm and the sub- 

 stage condenser mount. The condenser is shown in the mount, and it is 

 centered by means of the back pair of screws. The diaphragm cell is 

 screwed into the bottom of the mount, and the front, lower pair of 

 screws are used to center the diaphragm cell. R. and J. Beck, Ltd. 

 (1949), also provides a single-unit condenser mount. The phase 

 condenser may be obtained in either a fixed or centering mount. Each 

 condenser diaphragm is contained in a tray which is attached to the 

 substage condenser mount by the quick-change method. The dia- 

 phragm can be centered in its tray with respect to its corresponding 

 phase objective. If the diaphragm is centered and the tray is removed 

 and reinserted, recentering of the diaphragm is not required if the same 

 phase objective is used. C. Baker, Ltd., supplies a single-unit substage 



