niaii-powER AX I) nnuTisE MfCRosrnry loi 



12. To consider the objective of 1.4 aperture always i j ^ 

 better than the corresponding objective of 1.3 aperture. 

 The latter has sometimes better definition, especially if the 

 centering is not perfect. 



13. To use slides of differing thicknesses. These spoil 

 the corrections of the condenser, and thus lessen aperture. 

 Plate glass of fixed thickness (0.9 to 1.1 millimeters) is 

 readily obtainable. 



14. To use an immersion condenser dry. Its corrections 

 are thus spoiled. If water immersion is used, the process of 

 immersion is clean and easy. 



15. To use ordinary thick paraffin oil as immersion oil. 

 A trial showed that an apochromatic objective costing 

 $108 was thus reduced a perceptible degree below the level 

 of an achromatic objective which cost $25. 



16. To use a dry objective as a water immersion. The 

 laws of optics ensure bad images. 



17. To leave the condenser iris wide open for the low- 

 power objective. It will usually pay (by glare being 

 absent) to take out the eyepiece and look at the back of the 

 objective while closing the iris till the objective circle is 

 not quite full of light. 



18. To fail to lift a high-power objective before swinging 

 it from the slide. This sometimes results in injury to 

 the cover or the objective, especially with the objectives of 

 shortest focus, used on thickish covers. 



19. Bad centering of the objectives on the nosepiece 

 should not be allowed. It not only injures the images 

 of the object, but it makes focusing down of the high powers 

 on the small diaphragm image unreliable, 



20. A good nosepiece should apparently always be turned 

 one way (clockwise, as seen from below, with Zeiss 

 nosepieces). 



21. With covers varying from 0.17 millimeter, even 

 the oil-immersion objective will not give the optimum 

 image unless the tube length is correspondingly altered. 



22. When ordinary oil-immersion objectives are used 

 without a cover, the best image will not be got without a 

 corresponding increase of tube length. 



