190 ELEMENTARY CHEMICAL MICROSCOPY 



coordinate paper. This curve can then be employed in future 

 measurements. It is obvious that the nearer the scale is to the 

 microscope the greater will be the magnitude of the scale image, 

 and the farther the scale the smaller the graduations will appear. 

 Once the " curve " is obtained, we have at our command a 

 device for accurate measurements (for all save very minute 

 objects), by means of a scale the working magnitude of whose 

 divisions is variable at will between wide limits. 



This method of micrometry is especially convenient when 

 employing binocular microscopes or where special rulings are 

 required in quantitative work. The use of specially ruled glass 

 cells is thus avoided. 1 



Method 5. 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. 



1 Dr. W. W. Andrews of Regina, Canada, writes that he finds it possible to obtain 

 measurements of satisfactory accuracy by projecting the image of a window screen 

 into the plane of the object. The position of the microscope, at the time of cali- 

 bration, having been marked upon the work table top by drawing a pencil around 

 the base of the microscope. 



