USEFUL MICROSCOPE ACCESSORIES 115 



the stage micrometer just tangent to the circle of the field of 

 view, read the stage and displace the micrometer until the same 

 line is tangent to the field at the opposite end of the diameter 

 of the field-circle and again read the stage scale. The difference 

 in the readings will give the number of scale divisions necessary 

 to bring an entirely new area of the preparation into the field 

 of view with that particular optical combination which has been 

 employed. 



When the entire area of the preparation must be studied, the 

 student must of course look into the instrument while the prep- 

 aration is slowly displaced in one direction, as, for example, to 

 the right or left, and then turn the stage up or down the proper 

 number of scale divisions and again observe the slowly changing 

 field as it is displaced in the opposite direction to the left or 

 right. 



Rotating or Orienting Devices. It not infrequently happens 

 that irregular fragments of material must be carefully studied, 

 or that the exact relation of one surface to that adjacent to it 

 must be determined, or that the behavior of light rays sent 

 through the body in different directions be ascertained. To 

 facilitate the changing of the position of the substance and to 

 enable the worker to so place it that the surface being examined 

 shall lie in a plane normal to the optic axis, various orienting 

 devices have been suggested. 



The simplest of these consist of either metal or glass hemi- 

 spheres of such a size as to fit into the opening of the stage or into 

 the opening of a plate laid upon the stage ; the upper part of the 

 hemisphere is usually a truncated cone. Having a lower hemi- 

 spherical surface the apparatus may be tipped in any direction 

 and at any angle up to approximately 45 degrees. 



The Glass Hemisphere, as employed simply for the purpose 

 of facilitating the examination of irregular objects, is shown in 

 Fig. 58 ; a band around the hemisphere gg is rough ground so as 

 to prevent slipping when the device is tipped. The object o laid 

 upon the upper or flat surface can be so tipped as to permit the 

 different surfaces to be studied without difficulty. 



In certain classes of microscopes as, for example, Dennstedt's 



