12 THE USE OF THE MICROSCOPE 



9. Mount objects in immersion oil (Gelei, Metzner). 



10. Have cover-glasses of 0.16 to 0.17 millimeter (with all 

 important objects). Use a screw gage to measure them. 



11. Center lamp, ground glass, reflecting prism, accessory lens, 

 iris, lenses of condenser, and especially high-power objectives 

 and eyepieces, in the optic axis; and keep them centered. 



12. Have a binocular which allows of correct tube length being 

 employed (Siedentopf). Use the star test for this. 



13. Keep the magnification at, or not too far below, 1,000 

 times the working aperture (Abbe). 



Working 



1. Put distilled water between the slide and the condenser. 



2. Roughly focus the low-power objective on the slide. 



3. Focus and center the image of the ground glass on the 

 slide. 



4. Put a suitable (yellow-green) screen before the reflecting 

 prism. 



5. Focus the object accurately, after putting it in the small 

 disc of light on the slide (seen from outside the microscope). 



6. Cut down the aperture of the condenser to a ^{q cone or 

 less. Observe this by a 10-times magnifying lens held over the 

 eyepoint. 



7. Search the slide with a low-power objective. 



8. Put a 3-millimeter diaphragm on the source. Focus its 

 image on the selected part of the object, and center it. 



9. Change to the oil-immersion objective, and oil both the 

 slide and the objective. 



10. Turn down the microscope tube till the oil drops fuse. 

 Raise the objective slightly. Then focus down on the image of 

 the small diaphragm. 



11. Put the condenser iris to a %o cone or less. 



12. Accurately focus the image of the diaphragm, and accu- 

 rately center it. Then high-power observation may begin. 



(If the cover-glass thickness is not 0.17 millimeter, it should 

 have been measured with the micrometer screw, and the 

 tube length appropriately adjusted, even with oil-immersion 

 objectives.) 



Summary. — By not disregarding the optical rules 

 which govern the use of the microscope in scientific work, the 



