ORIENTATING DEVICES 141 



In order that full use may be made of a mechanical stage the 

 amount of displacement must be indicated by equivalent scales 

 on each of the two movements. It is therefore essential to find 

 the value and the uniformity of the scale divisions and to find the 

 diameter of the field of the microscope as indicated on the scale of 

 the stage. This may be accomplished by laying a stage microm- 

 eter in place between the clips F/ of the stage and measuring 

 the displacement under a cross-haired eyepiece for different por- 

 tions of each of the lateral scales of the stage. There is thus 

 ascertained the true value of the graduations, whether both 

 scales are equivalent and whether the scale divisions are of uni- 

 form size. To determine the amount of stage displacement 

 necessary to just include an entirely new area, bring a line of 

 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 Orientating Devices. It not infrequently hap- 

 pens 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 orientating 

 devices have been suggested. 



