328 COMPOUND AND ELECTRON MICROSCOPES 



aperture of the condenser or concave mirror. Thus, if a 4-mm objective 

 of N.A. 0.65 is used without an Abbe condenser but with the concave 

 mirror, a working numerical aperture equal to (0.65 + 0.25) /2 = 0.45 

 N.A. is obtained. Unquestionably a substage condenser gives better 

 results, but where no condenser is available the concave mirror limits the 

 cone of illumination. 



Draw Tube 



Some models of microscopes are furnished with draw tubes in place of 

 the fixed eyepiece tube adapter. The draw tube is graduated on the 

 side in millimeters and normally is set to be used at 160 mm. The 

 draw tube permits the use of long equivalent focus objectives without 

 the necessity for a length of rack for focussing that would be imprac- 

 ticable for most work. 



The draw tube serves three purposes. The first and most important 

 purpose is to compensate for small differences in cover-glass thickness 

 when high-power dry objectives are used. An 0.18-mm cover-glass 

 thickness (n = 1.520) is used in standard practice. If the cover glass 

 is not exactly 0.18 mm thick, increasing the tube length 10 mm will 

 compensate a decrease of 0.01 mm in cover-glass thickness. 



The second purpose is to permit small changes in initial magnifica- 

 tions so that, in calibrating micrometer eyepieces for purposes of meas- 

 uring, an even factor may be secured. 



The third purpose is to use it as a mount for low-power objectives. 

 In this way a lower magnification can be secured than if the same 

 objective were attached to the nosepiece. In effect, this is simply 

 using a low-power objective with a shorter optical and mechanical tube 

 length. 



Eyepieces 



A microscope ocular or eyepiece consists of one or more converging 

 lenses mounted at the top of the draw tube. It is used to magnify the 

 real image formed by the objective. It also serves to correct some of the 

 defects of the image produced by the objective. 



In the positive ocular (Ramsden's eyepiece) the real image to be 

 magnified by the ocular lies below the lens train in the plane of the 

 ocular diaphragm. A positive ocular can be identified by the position 

 of this diaphragm below the field lens. In its simplest form, Fig. VIII- 

 11, it consists of an eye lens (EL) and a field lens (FL) of equal focal 

 length, the distance between them being two thirds of the focal length 

 of either lens. It is equivalent to a single lens whose focal length is 



