MICROMETRY MICROMETRIC MICROSCOPES 157 



making a positive from the negative thus obtained. This screen 

 is held in a vertical position for use in transverse slots cut in a 

 long, narrow block of wood which is notched at the end so as to 

 permit its being always placed in exactly the same position 

 against the base of the microscope; the glass screen carrying 

 the scale is illuminated from the back and the mirror of the micro- 

 scope turned and tipped so as to throw an image of the scale 

 into the condenser. Raising or lowering the condenser focuses 

 the scale upon the object. 



The value of the image of the scale is determined by means of 

 a stage micrometer and the position of the screen in the block 

 (i.e., its distance from the Abbe condenser) recorded. The 

 nearer the scale-screen to the condenser the larger the image of 

 the rulings and the farther the scale the smaller the image. To 

 obtain the best results illuminate the screen with artificial light 

 placed behind ground glass. 



Method 6. Micrometry by means of the fine adjustment 

 micrometer screw. Most microscopes are provided with a fine 

 adjustment so constructed with micrometer screw, accurately 

 ground wedge or cone as to permit measurements of the thick- 

 ness of objects through a determination of the amount of dis- 

 placement necessary to focus the instrument upon the lower 

 and the upper surface of the object. The amount of displace- 

 ment is indicated by a graduated head or drum attached to the 

 fine adjustment moving past a fixed index. 



The value of one scale division of the drum is usually marked 

 by the maker upon the instrument or indicated upon the table 

 of magnifications accompanying the microscope when purchased. 

 If this value is unknown it may be ascertained by placing an 

 object of known thickness having parallel sides upon an object 

 slide, clamping as tightly as possible to the slide with the stage 

 clips and focusing first upon the slide, then upon the upper sur- 

 face of the object. The difference in the fine adjustment drum 

 readings will give the number of divisions equivalent to the 

 thickness of the object. The thickness of the object used may be 

 determined by placing it edgewise on the stage and measuring 

 its thickness by any one of the micrometric methods given above. 



