137 



mirror. Now use the micrometer screw, remem- 

 bering to grasp the milled edge between the 

 thumb and forefinger, and work to the right and 

 left, to reach different depths, and note carefully 

 the beautiful structure which is open to view. 



Opaque Object. We will now suppose that 

 the object is a mineral or plant. Place this upon 

 a slide and slip under the clips. In this case the 

 low power objective is used for two reasons ; 

 because a general view is sought, involving low 

 magnification and large field with light-giving 

 power and if it should be desired to use a higher 

 power this cannot be done on account of the short 

 working distance. The light may and undoubt- 

 edly will be found insufficient to distinguish the 

 object clearly. If the instrument is of the 

 American type, swing the mirror-bar upon its axis 

 around the stage to a point above it so that it will 

 be at an angle of about 45 to its surface. If a 

 lamp is used and is in the same position as when 

 used for transmitted light, it is probable that the 

 tube of the instrument will obstruct the light and 

 it is then well to move it toward the front. Using 

 the concave mirror, adjust it so that the light will 

 be concentrated upon the object, by watching it 

 directly, and then observe through the tube. If it 

 is not sufficiently illuminated continue to adjust 

 the mirror ; also vary its distance from the object 

 and swing the mirror-bar to a higher or lower 

 point, 



