60 APERTURE OF THE SUBSTAGE CONDENSER [Cn. II 



105. Testing the centering by slowly opening the diaphragm. - 

 After the condenser is centered as well as possible with the small dia- 

 phragm, one can test its centering rigidly by looking directly down the 

 tube without the ocular or by looking at the eye-point with a magnifier 

 and slowly opening the diaphragm. If it is accurately centered, the 

 black ring will become narrower and disappear all around at the same 

 time. If it is not accurately centered the black ring will open on one 

 side. In case the condenser is not found centered, change its position 

 slightly until the black ring disappears at the same time all around. 



106. Numerical aperture of the condenser. As stated above, 

 the aperture of the condenser should have a range to meet the require- 

 ments of all objectives from the lowest to those of the highest aperture. 

 It is found in practice that for diatoms, etc., the best images are 

 obtained when the object is lighted with a cone which fills about three- 

 fourths of the diameter of the back lens of the objective with light, 

 but for histological and other preparations of lower refractive power 

 only one-half or one-third the aperture often gives the most satis- 

 factory images. 



To determine this in any case focus upon some very transparent 

 object, take out the ocular, look down the tube at the back lens or 

 look at the eye-point (fig. 22-24) with a magnifier. If less than three- 

 fourths of the back lens is lighted, increase the opening in the 

 diaphragm if more than three-fourths, diminish it. For some 

 objects it is advantageous to use less than three-fourths of the aper- 

 ture. Experience will teach the best lighting for special cases. 



107. Aperture and source of light. As shown by Brewster 

 nearly a century ago, and as brought out in late years by Gordon and 

 Wright, the amount of aperture giving the best results depends on the 

 size of the source of light. In 106 it was assumed that a large source 

 was used like a window or door. If now a small source like the illu- 

 minated disc of daylight glass in the lantern for artificial daylight 

 (fig. 37-38) is used, then considerably more of the aperture of the ob- 

 jective can be utilized without the fogging and loss of detail which 

 results when using a large source like a window. It is also a funda- 

 mental fact that the larger the aperture utilized, the more details, and 

 the more sharply are the details brought out in the specimen. 



