1 12 THE USE OF THE MICROSCOPE 



test, used with an Abbe test plate. Hence, if the tube 

 length can be shifted through the interval from 200 to 

 150 millimeters (which is not unusual), and if the objective 

 is calculated for 0.17 millimeter of cover-glass thickness, 

 correction can be made for covers from 0.13 to 0.18 milh- 

 meter by this means. (The 20 objective needs only about 

 2 millimeters increase for every 0.01 millimeter of cover- 

 glass deficit.) 



4. If a 40 objective with a correction collar is purchased, 

 correction can then be made for cover-glass thicknesses 

 from about 0.10 to 0.20 millimeter. In this case it is usually 

 best to measure the covers with a special gage (which is 

 the quickest way). This measuring may also be done with 

 the micrometer screw of the microscope and a dry objective; 

 multiplying in this case by 1.5, because the refraction of 

 the glass makes it seem thinner. 



5. Another remedy is to replace the dry 40 objective by 

 a medium-aperture, oil-immersion objective, which gives 

 nearly equally good images of balsam-mounted objects with 

 all the usual cover-glass thicknesses. 



6. A 40 water-immersion objective is made by one optical 

 firm, which shows sharp definition in the center of the field 

 with all ordinary thicknesses of cover-glass. It is of 0.75 

 aperture. 



7. The 40 dry objective may be omitted from the outfit, 

 and a 20 objective used in its place. This is less sensitive 

 to slight differences of cover-glass thickness, and is more 

 readily corrected by altering the tube length. 



8. Cover-glasses may be omitted in all cases, and special 

 dry and oil-immersion objectives employed, corrected for 

 use without a cover-glass. 



That some workers in histology are desirous that they 

 should obtain from their microscopes as good images as the 

 objectives were made to give, is shown by Professor 

 Schmidt's article in the Biologische Centralblatt, for 1922, 

 in which he describes the state of affairs which has come 

 under his attention with regard to the cover-glass problem 

 (113). 



