782 SUMMARY OF CURRENT RESEARCHES RELATING TO 
B lies firmly upon the table C, and contains at b b two short pins which 
rest firmly in the two spiral grooves. “ These two pins will direct the 
motion applied; the section describing exactly similar revolutions, as 
the pins b b, will bring exactly the width between the lines of the spiral 
groove under the focus as a definite part of the same, decreasing in 
diameter as the power of magnification increases. The diameter of the 
field and the space between the lines of the spiral groove can correspond 
exactly only with the power for which the instrument has been made, 
and the possibilities lie between 50 and 250 diameter magnification. 
To find the actual field, moasure the diameter of the field in millimetres 
and divide this by the previously ascertained magnification. The 
diameter of the actual field will be the exact distance between the lines 
of the spiral groove. In most instruments, 100 to 150 diameters give 
fields of 1 mm. diameter, just wide enough for the grooves, while the 
powers beyond these furnish too small a space. The apparatus should 
therefore be exactly fitted for one of the lower powers of the Micro- 
scope. C, the table, is firmly but not permanently attached to the 
stage of the Microscope by means of pins and sockets, clamps or screws, 
according to the stage of the instrument.” 
Fig. 109 
Desk for Microscopical Drawing.*— Dr. W. Bernhard has designed 
a new desk for microscopical drawing which in many respects is an 
* Zeitschr. f. wiss. Mikr. , ix. (1893) pp. 439-45. 
