CENTERING TELESCOPES 127 



phase microscope stand equipped with a long working distance condenser 

 in the single-unit substage mount. This condenser permits a working 

 distance in the range 9.5 mm to 10 mm in air (more in glass or water) 

 when the condenser is adjusted for Kohler illumination. 



The American Optical Co. supplies long working distance condenser 

 elements and diaphragms which fit into its turret substage mount. A 

 condenser consisting of two lenses is used with the phase objective of 

 4-mm focal length, and a working distance of approximately 7 mm in 

 air is obtained. Only the bottom lens element of this condenser is 

 retained with the phase objectives of 8-mm and 16-mm focal length, 

 and the working distance is approximately 8 mm in air. This condenser 

 lens is also the bottom element of the standard phase microscope con- 

 denser, N.A. 1.25. The diaphragms are contained in individual cells 

 which screw into the rotatable changer plate of the turret mount. Of 

 the phase microscope condensers supplied by various manufacturers as 

 standard equipment with the rotatable disk type of substage condenser 

 mount, some of the condensers having a numerical aperture of 1.0 or 

 less are also suitable for slides of greater than standard thickness 

 (e.g., five times the standard glass thickness). The phase microscope 

 condensers made by Zeiss, Jena and by Cooke, Troughton and Simms, 

 Ltd., are examples. Cooke, Troughton and Simms, Ltd., also offers a 

 special condenser which permits a working distance of 6 mm in air. 

 Koristka and Co. lists a special, long focal length condenser for its 

 "simplified" phase microscope. 



4.3. Centering telescopes 



All manufacturers mentioned in the preceding part of this section, 

 with the exception of Officine Galileo and L Koristka and Co., supply 

 separate, adjustable centering telescopes (long-focus microscopes, in 

 fact) which can be inserted in the body tube in place of the eyepiece and 

 focused on the coating of the diffraction plate. In the phase microscope 

 made by the firm of Galileo, the eyepiece is not removed, and the user 

 operates a knob on the body tube to place a lens in the path of light 

 below the eyepiece. The eyepiece together with this lens forms the 

 centering telescope. The knob also controls a focusing adjustment so 

 that the centering telescope can be focused on the coating of the diffrac- 

 tion plate. In the Koristka phase microscope, a centering ocular 

 replaces the eyepiece of the microscope and a knob is turned to move a 

 second lens into the light beam so that these two components become the 

 centering telescope. 



