INTRODUCTION 



Causes of Injury. — ■(Continued) 



Cause 



7. Light not best color. 



8. Double source of 



light (direct light 

 from lamp plus 

 light diffracted 

 from ground 

 glass). 



9. Source of light out 



of center line. 



10. Near source of light 



used with con- 

 denser corrected for 

 parallel rays. 



11. Multiple image of 



source from mir- 

 ror, with small 

 diaphragm. 



12. Mirror (or reflecting 



prism) out of 

 optic axis. 



13. Condenser not cor- 



rected for spherical 

 aberrations. 



14. Condenser aspheric, 



but with all chro- 

 matic aberrations. 



15. Condenser out of 



focus. 



16. Condenser not cen- 



tered in optic 

 axis. 



17. Top lens of dry or 

 water-immersion 

 condenser soiled 

 with immersion 

 oU. 



Injury 



Does not improve cor- 

 rections of lenses; or 

 does not show true 

 colors. 



The source which is 

 unfocused produces 

 glare. 



Obliquity of image of 

 source, especially in- 

 jurious with dark 

 field. 



Spherical and chro- 

 matic aberrations, 

 causing loss of aper- 

 ture. 



Causing difficulty in 

 focusing, and some 

 glare. 



Obliquity of illumina- 

 tion, especially in- 

 jurious with dark 

 field. Also, often, 

 loss of aperture. 



Glare, and loss of aper- 

 ture. 



Glare, and lessened 

 aperture. 



Glare, and loss of aper- 

 ture. 



Obliquity of illumina- 

 tion, especially in- 

 jurious with dark 

 field. Also causes 

 loss of light. 



Loss of light, and some 

 glare. Water will 

 not cling. 



Preventive 



Use yellow-green screen 

 for the first; and day- 

 light glass for the sec- 

 ond. 



Make ground glass 

 opaque enough to 

 exclude direct light. 



Put source of light in 

 center front. 



Use achromatic acces- 

 sory lens with con- 

 denser corrected for 

 parallel rays. 



Use reflecting prism, 

 silvered if necessarv. 



Fix mirror or prism 

 central. 



Use aspheric or achro- 

 matic condenser. 



Use fairly deep yellow- 

 green light filter, such 

 as No. 58 of the 

 Wratten screens. 



Focus condenser on ob- 

 ject. 



Should be centered by 

 maker, or centering 

 device provided. 



Clean with xylol. 



