250 THE USE OF THE MICROSCOPE 



The 2- or 3-millimcter diaphragm on the source of hght is 

 to be focused together with the object; first by the 10 

 objective of 0.3 aperture, then by the high dry objective 

 of 0.85 aperture, and finally by an oil-immersion objective 

 of 1.3 aperture; a slide of standard thickness and a standard 

 cover being used, with a delicate well-stained object. In 

 each case, employ the largest cone that can be obtained 

 without glare. It will be found that the condenser has to 

 be raised to focus the margin of the diaphragm as the 

 change is made to an objective of higher aperture. This 

 is, of course, because the outer zones of the condenser have 

 a shorter focus than the central ones, since the condenser 

 is undercorrected. For overcorrection, which is not so 

 commonly met with, the reverse is the case. Only when 

 the corrections are fairly complete do the zones focus 

 together. (Nelson.) 



8. Marginal Ring Test for Correction of Condenser. — 

 Arrange condenser, etc., as in 7. Use a 100 fluorite (or 

 90 apochromatic) objective of 1.3 aperture, and a 2- or 

 3-millimeter diaphragm on the source. Have an oil- 

 immersion condenser, corrected for parallel rays. Use 

 it as a water immersion, with a source of light 25 centi- 

 meters distant. On looking at the back of the objective, 

 with both condenser and objective focused, and the iris 

 opened fully, a marginal ring is visible besides the central 

 disc, when the condenser is raised. This becomes more 

 pronounced when the condenser is raised more, and dis- 

 appears when it is lowered beneath the focus. (An over- 

 corrected condenser shows such a marginal ring only when 

 the condenser is lowered.) The writer regards this as the 

 most practical test for the adjustment of the condenser. 

 The correctly adjusted condenser shows no marginal 

 ring, either on raising or lowering; and has maximum aper- 

 ture with a small diaphragm. Poorly corrected well- 

 adjusted condensers may show strong marginal rings both 

 on raising and lowering, the central zone only being correct. 

 (Nelson, Coles.) 



