Ch. II] DETERMINING THE THICKNESS OF THE COVER 43 



§ 81. Demonstration that the working distance is lessened f 

 the thickness of the cover-glass. -- Use a clean, flat glass slide. Put 

 an ink or pencil mark on the upper face for object. Employ a 16 mm. 

 objective and 8x or iox ocular. Focus the microscope on the ink or 

 pencil mark, then measure the free space between the slide and the 

 end of the objective with the wooden wedge, as directed in § 78. This 

 is the free working distance (§ 77) without a cover-glass. 



Cut a glass slide up into two or three pieces for cover-glasses. Meas- 

 ure the thickness of one of the pieces with the cover-glass measurer or 

 in some. other good way. Place this over the mark on the slide which 

 was in focus. If now one looks into the microscope the mark will 

 not be in focus with the glass cover over it. Focus up carefully until 

 the mark is again in focus. Measure the space between the top of the 

 cover-glass and the objective with the work as before. This will 

 represent the free working distance with this cover-glass. 



Subtract the free working distance with this cover-glass from .that 

 with no cover-glass and the difference will be the amount the free 

 working distance has been lessened by the addition of the cover. 

 This amount compared with the thickness of the cover-glass will 

 give the ratio of lessening of working distance by the addition of the 

 cover-glass. 



In an actual case the results were as follows: 



Free working distance without cover 4.62 mm. 



" " " with cover 3.54 mm. 



Lessening of the working distance by the cover-glass 1.08 mm. 



The actual thickness of the cover-glass was 1.62 mm. 



That is, the lessening of the free working distance was not so great 

 as the thickness of the cover (1.62 mm.), but less; viz. 1.08 mm.; 

 that is, in the proportion of ~f = § of the actual thickness of the 

 cover-glass. 



§ 82. Determining the thickness of the cover-glass with mounted 

 objects. —From what has been learned about the free working dis- 

 tance with covered objects, it is possible to determine the thickness 

 of the cover-glass over an object if the object is in contact with the 

 cover. If it is below, as shown in fig. 32, and the mounting medium 

 is Canada balsam with approximately the same refractive index as 



