74 ELEMENTARY CHEMICAL MICROSCOPY 



eyes open should see the field as a single bright circle. If two 

 overlapping circles appear the oculars are too far apart. If 

 the field is blurred and both eyes cannot simultaneously see the 

 field, the oculars are too close together. A shutter which 

 automatically remains open, operated by a lever, s, is fitted 

 below the prism chambers in order to assist in testing whether 

 the proper pupillary distance has been secured. The observer 

 looks into the instrument with both eyes, turns the lever s first 

 to one side then to the other without moving the head. In this 

 way it can be ascertained whether both eyes are in actual use. 

 The shutter also serves in adjusting the focus of the paired objec- 

 tives. In the higher powered objectives one of each pair is 

 provided with a milled focusing collar m (on the right side). 

 The worker places a suitable object on the stage and focuses 

 the instrument; the shutter is then turned so as to cut off the 

 view through the right half of the objective and the microscope 

 is very carefully focused. The shutter is next turned so as to 

 cut off the left half which has just been focused and if the image 

 is not seen with equal clearness the focusing collar is turned 

 until the image becomes clear and distinct. The instrument 

 has now been focused for each eye and upon looking into the 

 microscope with both eyes the object being studied should stand 

 out stereoscopically and the image be clear and distinct. 



The mounting to which the two prism chambers are attached 

 can be rotated in order that the worker may look into the instru- 

 ment from the sides or front as the exigencies of the work may 

 demand. This arrangement adds greatly to the value of the 

 instrument. 



The magnifications available with this type of microscope 

 lie between about 10 diameters and 150 diameters, with free 

 working distances ranging from 70 mm. with the lowest power 

 to 25 mm. with the highest power. This is more than ample 

 to permit working with a variety of tools upon objects lying 

 on the stage. Hand rests (removable) attached to the stage 

 greatly facilitate manipulations. 



Two interchangeable stages are provided with each instru- 

 ment, one of glass the other of metal; the opening in the metal 



