376 OSCAR W. RICHARDS 



Fine adjustments of microscopes are calibrated for measuring 

 vertical distances, but limitations arise in the depth of focus of the 

 objective and the precision of the fine adjustment. To avoid hys- 

 teresis and other errors in the mechanism, the measurement must 

 always be made in the same direction, i.e., either up or down. When 

 precision measurement is required, the fine adjustment should be 

 calibrated with an interferometer {1, p. 194). The additional use of 

 long levers and protractors to give finer readings or movements may 

 be misleading in that they can exceed the precision of the fine ad- 

 justment. When the specimen is in a medium of different refractive 

 index than air the apparent thickness must be multiplied by the index 

 of the medium to get the tine thickness. 



When a suspension containing particles is placed in a cell of known 

 thickness, counts per unit volume may be made from knowing the 

 area of the field of view or with a net ocular reticule. The Whipple 

 disc is useful both for counts and for estimating percentage composi- 

 tion. Another method uses a chamber of known depth that has a 

 scale ruled on its base as in the familiar hemocytometer used for 

 blood counting. Various rulings are available and the chambers may 

 be purchased with various depths. Counting and measuring require 

 care, careful calibration, and proper enumeration and evaluation of 

 possible errors. Studies on the errors of blood counting are useful in 

 suggesting appropriate procedures for other types of counts {55). 



4. Drawing, Projection, and Records 



Good records are a necessary part of any research and those from 

 observations with the microscope may be made by drawing or photog- 

 raphy. A prism or a mirror can be mounted over the ocular, the 

 image projected onto a drawing surface and the outline filled in with 

 as much detail as desired. The "camera lucida" may be used to 

 view the drawing surface and the microscope image simultaneously 

 and is helpful for drawing records. The drawing surfaces should be 

 level to avoid distortion. It is desirable to test the drawing arrange- 

 ment by sketching a hemocytometer ruhng, stage micrometer, or 

 other known pattern to make certain that distortion is avoided. 

 This is important when the outlines are made on wax plates or other 

 materials that are to be combined later to give solid reconstruction 

 models {4,56). 



Photomicrographic records can be made usinr; a light-sensitive 

 plate or film instead of drawing paper and a canun-a or other dark 



