INTRObUCTIUN 



11 



Causes of Injury. — '(Continued) 



Cause 



57. Eyes not accommo- 



dated for dis- 

 tance. 



58. Eyes icith astigma- 



tism. 



59. Extraneous light in 



observing eye or 

 eyes. 



60. Unutilized eye at the 



monocular causing 

 disturbance. 



61. Soiled spectacle 



glasses. 



62. Spectacle glasses 



not at right angles 

 to the optic axis. 



Injury 



Slightly wrong position 

 of image in eyepiece. 



Final image weak in 



some azimuth. 

 Glare. 



Accommodating, con- 

 verging and con- 

 tracting troubles. 

 Fog. 



Injuries to best defini- 

 tion. 



Preventive 

 Use distance spectacles. 



Use correcting glasses. 



Put shield in front of 

 eyepieces. 



Translucent screen be- 

 fore unutilized eye. 



Keep spectacles opti- 

 cally clean, as part 

 of the microscope. 



Incline microscope ap- 

 propriately. 



SOME RULES FOR OPTIMUM HIGH-POWER MICROSCOPY 



Apparatus 



1. Have, as radiant for bright field, a glass plate finely- 

 ground on both sides, together with a powerful enough electric 

 lamp (Hartridge). Focus the incandescent tungsten ribbon for 

 dark field. 



2. With high-power objectives, put a 3-miUimeter dia- 

 phragm close to the ground glass (Beck). This cuts off glare. 



3. Adjust lamp distance to suit corrections of condenser 

 (Ainslie, Hartridge), using the ring test. This adjustment is 

 important. 



4. Use (neutral screens, or) yellow-green light filters, to 

 moderate the illumination (Barnard), and improve the definition 

 (Spitta). 



5. Employ a (silvered) reflecting prism, instead of a glass 

 mirror (Dallinger). 



6. Center a suitable correcting achromatic lens immediately 

 below the condenser (Hartridge), if the condenser is made for 

 parallel rays. 



7. Have a corrected achromatic condenser of nearly 1.3 true 

 aperture, and use it water immersed (Nelson, Hartridge). 



8. Keep slides to 1 millimeter thickness, within 0.1 milli- 

 meter (Hartridge). Such slides are readily procurable. 



