Microscope Construction, Use, and Care 197 



microscope for most work. However, if a fluid mount is used on a 

 tilted stage, disturbing currents are likely to be set up in the litjuid, 

 and the liquid might drain into the diaphragm; therefore, it is 

 advisable to use wet preparations on a horizontal stage. To forestall 

 the progressive trend of weary students toward a reclining position, a 

 hinge stop can be installed on modern microscopes, preventing tilting 

 beyond 30° (Fig. 16.3) . 



Micrometry 



The measurement of minute objects by means of the microscope is 

 an interesting and valuable feature of microscopic study. Although 

 the procedure is simple and rapid, the method does not receive 

 adequate attention in teaching. The simplest form of measuring 

 device is an eyepiece micrometer, a disk of glass having an engraved 

 scale, a series of accurately spaced lines. The spaces do not have a 

 standard value, and each disk must be calibrated for each given ocular 

 and set of objectives. Place the disk upon the metal diaphragm in the 

 ocular. If the diaphragm is in the correct position, the lines on the 

 disk will be in sharp focus. Occasionally, these diaphragms become 

 displaced, but they can be pushed back and forth with a softwood 

 stick until the eyepiece micrometer is in foctis. 



The stage micrometer with which the calibration is made is a slide 

 bearing an engraved scale with known values, usually in tenths and 



Stage micrometer scale 



13.6^ 



I HI 



O.lmm. 



0.01mm. 



Eyepiece scale | 

 -- 50 divisions -^ 



=0.58 mm. 



Fig. 16.4-Calibration of an eyepiece micrometer disk and measurement of a 



minute object. 



hundredths of a millimeter, but scales in hundredths of an inch are 

 obtainable. When the stage micrometer is brought into focus, the 

 scale of the eyepiece will be seen superimposed on the scale of the 

 stage micrometer. Shift the stage micrometer and revolve the ocular 

 until the two scales are in such position that the values may be 

 compared. A specific case using a 43x objective and a lOX ocular is 

 shown in Fig. 16.4. It will be seen that the 50 small divisions of the 



