374 OSCAR W. RICHARDS 



affect the image and it is preferable in critical microscopy to correct 

 for any residual aberration. 



When a specimen is mounted in a denser medium than air, a 

 medium of the same refractive index should be used both bet^^■een 

 the condenser and the slide and between the cover glass and the ob- 

 jective; otherwise, the numerical aperture may be limited to unity 

 with a corresponding restriction on the resolution. For objectives of 

 greater aperture, the immersion fluid for which they were designed 

 (water, glycerin, or an oil of the same index as the glass) must be 

 used (Fig. 6A-B). 



During the early nineteenth century catoptric, or reflecting, ob- 

 jectives were preferred to those made with lenses. Curved mirrors 

 were used in place of lenses. At that time achromatic lenses were 

 not yet available and microscopists probably could do better with the 

 reflecting objectives. Mirrors do not have chromatic aberration and 

 so are in focus for infrared light and for ultraviolet. The use of mir- 

 rors of aspheric shape avoids spherical aberration and wdth a suit- 

 able metal surface short wavelength radiation may be used with a 

 corresponding gain in resolution. Some light loss occurs at the open- 

 ing in the mirror to pass the light on to the ocular, or from the second 

 mirror used in the system. At present there is a return in interest to 

 the possibilities of reflecting microscopes (19,48-51). 



The binocular body is more comfortable, since the work of seeing 

 is shared by both eyes, but does not usually give a stereoscopic image, 

 because each eye sees the same identical image. A stereoscopic ef- 

 fect of depth may be obtained (1) by setting the interpupillary dis- 

 tance so that the iris of each eye cuts off half of the light from the ocu- 

 lar, (^) by placing stops within the prism system to accomplish the 

 same effect, or (3) by using D-shaped stops over the eye points (Rams- 

 den discs) of the oculars. Other methods involve placing a prism over 

 a lateral half of the back aperture of the objective, so arranged that 

 the light from each half goes to opposite eyes, or by a Polaroid disc 

 with lateral halves having polarizing axes at 90° to each other and 

 Polaroid eyepiece filters adjusted so that each eye sees the light com- 

 ing from one-half of the objective (58). The half-aperture methods 

 reduce the resolving power by one-half, which may or may not be 

 significant, depending on the fineness of detail being observed. 



Huygenian oculars give a bright image that has distortion at the 

 edge of the field and should not be used for measuring and drawing of 

 detail much beyond the center of the field. Compensating oculars 



