8 



THE USE OF THE MICROSCOPE 



Causes of Injury. — ■(Continued) 



Cause 



18. Iris of condenser 



open too much. 



19. Iris of condenser not 



open enough. 



20. Immersion condenser 



not immersed. 



21. Slide too thick, ivith 



dry or water-im- 

 mersion condenser. 



22. Slide too thin, with 



dry or water-im- 

 mersion conden- 

 ser. 



23. Object in water, or 



watery medium. 



24. Object mounted dry 



on the slide, under 

 cover-glass. 



25. Objects in media of 



refraction over 

 1.52, such as 

 hyrax. 

 26. Objects in media 

 other than air, 

 water, and im- 

 mersion oil, re- 

 spectively. 



27. Object and medium 



close in refractive 

 power. 



28. Objects lightly 



stained, or natu- 

 rally feebly col- 

 ored. 



29. Cover-glass not op- 



tically clean on 

 under side. 



30. Cover-glass too thick; 



over 0.17 milli- 



Injury 



Light-flood (bright field 

 plus dark field). 



Loss of aperture. A 

 ^i'o cone should be 

 aimed at, if possible. 



Loss of aperture, and 

 loss of light. Spher- 

 ical errors. 



Overcorrected spher- 

 ical errors of con- 

 denser, with loss of 

 aperture. 



Undercorrected spher- 

 ical errors of con- 

 denser, with loss of 

 aperture. 



Fogged images with 

 oil-immersion objec- 

 tives, except close to 

 cover-glass. 



Fogged images with 

 oil or water-immer- 

 sion objectives. 



Errors with oil-immer- 

 sion objectives, if a 

 few microns below 

 the cover. 



Spherical and chro- 

 matic errors, increas- 

 ing with thickness of 

 medium. 



Little diffraction, and 

 consequently poor 

 visibility. 



Low visibility with 

 large aperture. 



Preventive 



Restrict condenser aper" 

 ture to less than aper- 

 ture of objective. 



Open as widely as glare 

 will allow. 



Immerse in water, or 

 cedar oil. 



Use slides 1 millimeter 

 thick; or put lamp 

 closer. 



Use slides 1 millimeter 

 thick; or put lamp 

 farther off ; or immerse 

 with xylol instead of 

 water. 



Use water-immersion 

 objective, with cor- 

 rection collar. 



Use dry objective, and 

 sometimes dark field. 



Mount only on cover- 

 glass. 



Mount only in these 

 three media; or 

 ])artly correct by 

 altering tube length. 



Use annular illumina- 

 tion, or dark field. 



Use appropriate com- 

 plement a r y c o 1 o r 

 screens, or dark field. 



Fog, worst with dark Optical cleanliness of 



field. covers. 



Spherical overcorrec- Use measured covers, 



tion of drv and 0.17 millimoter tliick. 



