146 THE USE OF THE MICROSCOPE 



7. The low objectives in the nosepiece should be made fairly 

 concentric with the high ones, which must be accurately centered. 

 If not, all images are impaired. Special centering rings are now 

 available. Sliding objective changers make centering easy. 



8. When the objectives are properly concentric, and with a 

 small diaphragm on the source of light, the high objectives can be 

 focused down on the circle of light (on the object), instead of 

 being focused up. 



9. The tube length should be accurate to within 1 millimeter 

 with oil-immersion objectives. 



10. Pull out the drawtube an appropriate distance with 

 uncovered objects for the 16-, 11- and 8-millimeter objectives, 

 (and also for the oil-immersion objectives). 



11. These three dry objectives, and especially the 4.3-milli- 

 meter dry objective, and also the oil-immersion objectives, can 

 be adjusted by the drawtube for the usual differences in cover 

 thickness (or the eyepieces can be pulled out, on the binocular). 



12. With oil-immersion objectives used on objects in water, 

 also pull out the drawtube sufficiently. 



13. Put a diaphragm of about 14 millimeters in diameter in the 

 drawtube well below the eyepiece especially when the source of 

 light is large (Nelson). 



14. For most routine work, have the drawtube fixed perma- 

 nently at the right length by a special screw, or use an instrument 

 without drawtube. Correct covers are better than correction by 

 drawtube. 



