PRACTICAL EXERCISES ]VI'ril THE MICROSCOPE 259 



(lisplaocmcnt of llio zero, or wipc^ (he i)laiie faces and shift 

 the regulating screws until the mark consistently comes at 

 the zero point of the scale. Then measure the cover, say 

 0.15 millimeter. Use it to cover a thin object in immersion 

 oil or balsam. Measure it again on the slide by focusing 

 with a dry objective by the micrometer milled head of 

 the microscope from particles on the lower surface of the 

 cover to dust on the upper surface. The result will be 

 about 50 if the divisions are 2 microns each (Zeiss) or 100 

 if the divisions on the scale are 1 micron each. Hence 

 multiply by 3 in the former, and 1.5 in the latter case, and 

 the cover thickness is given in microns. (An oil-immersion 

 objective gives the cover thickness, or any measurement 

 of depth in immersion oil, correctly without multiplication. 

 Test this.) Set the correction collar of the water-immer- 

 sion objective at the ascertained figure of cover thickness, 

 and focus the objective. Shift the collar first one and then 

 two grades each way, and see if the image is improved. 

 Large covers sometimes differ 10 microns in thickness in 

 different parts. 



27. To Find the Proper Cover-glass Thickness to Use 

 with a High Dry Objective. — Take an Abbe test plate, 

 which has a long, narrow cover-glass, of thickness varying 

 from under 0.10 to over 0.20 millimeter. On the under 

 side, about four groups of parallel lines have been engraved 

 in a silver film with a broad band of silver between each 

 group. Take a dry 40 objective of 0.85 aperture, without 

 a correcting collar. Use a %o cone from a well-corrected 

 and well-adjusted condenser, and a 20 compensating eye- 

 piece. Focus the edges of the silver lines, using a yellow- 

 green screen, say, No. 56 of the Wratten gelatin light filters. 

 Try which part of the long cover-glass gives the best 

 definition of the broken edges and granular structure of the 

 silver film at the center of the field. Note the correspond- 

 ing thickness of the cover marked on the shde. 



Look for some minute holes in the broader bands of 

 silver, and try the star test with these. The rule is: 

 diffraction mist when the object is beyond the focus of the 



