218 Botanical Microtechnique 



ocular, adjust the magnifier until the back lens of the objective is 

 in focus. Close the substage diaphragm and open it slowly until the 

 rim of the diaphragm coincides with the rim of the back lens of 

 the objective. 1 he full numerical ajjerture of the objective is utilized 

 only imder these conditions. In practice, the aperture may be reduced 

 by means of the substage diaphragm, but not more than one-sixth 

 of the diameter of the back lens of the objective. 



Up to this point the operations are identical for vistial study and 

 photography, and the foregoing operations can be performed with 

 the binocular body. This is the time to try Wratten filters — usually 

 in pairs — to obtain the desired contrast or detail. A neutral filter 

 may also be needed for visual work. 



7. If a binocular body is in place, replace it with a monocular 

 tube and connect with the camera. Remove all filters and arrange the 

 composition of the essential image on the ground-glass screen by 

 means of a revolving stage or revolving camera. 



8. Replace the selected Wratten filters (no neutral filter) , and 

 focus critically. Close the shutter, insert a loaded plateholder, wdth- 

 draw^ the slide from the plateholder, and make the exposure. (See 

 the discussion of exposure meters, page 213) . 



The foregoing procedure is modified with cameras that have an 

 observation tube (Figs. 17.1, 17.2). Composition and focusing can be 

 accomplished after the shutter has been closed, the film holder 

 inserted and the dark slide withdrawn. The prism or mirror that 

 serves the observation tube is then swung aside and the exposme 

 made. 



The lowest power objectives, such as the 3.2 X 'ii^<-l J^X' <-l^^ ii^t 

 cover a large enough field, and have too much magnification for some 

 sidjjects. The objectives of stereoscopic binocular microscopes have 

 the desired specifications, but such objectives are not adapted for 

 use on a single-tube microscope of correct tube length. The compoinid 

 system is therefore not suitable for photomicrographs in the 5X to 

 30X I'lnge (See frontispiece) . Such photographs are taken with 

 special objectives that produce a flat, well corrected image and are 

 never used with an ocular. Objectives of this type are the Micro 

 Tessars of Rausch and Lomb and the Micro-Teleplats of Spencer 

 (American Optical Co.) . Leil/ and Zeiss also make excellent objec- 

 tives of this type. Each Micro lY\ssar must be used with a condenser 

 that has the same focal length as the objective. The manufactmers 

 furnish matching condensers lor their objectives. The ilhiminani 



