72 THE A B C OF PHOTO - MICROGRAPHY 



and carefully bringing it into focus upon" the 

 object find that the brilliant line of light, as 

 seen under the lower power, has now broad- 

 ened out, covering the whole field of view, 

 giving us "critical" illumination, i.e., radiant 

 and object in focus together. The substage 

 condenser is the same achromatic one we have 

 employed before and we use it with nearly full 

 aperture ; this objective being equal to almost 

 any cone of light that may be offered to it. 

 The same aplanatic ocular is also in place, so 

 that our optical arrangements are unchanged, 

 save in the objective and larger stop of con- 

 denser. The plate chosen orthochromatic 

 of course is a Forbes slow L. The speci- 

 men shows several shades of red in the stain- 

 ing from a deep one to delicate pinkish. It is 

 evident that sufficient exposure to obtain de- 

 tails in the dark karyokinetic figure must in- 

 evitably obliterate the more lightly stained cor- 

 puscles unless restrained in some manner. This 

 we do with an acid-green screen placed in the 

 holder as shown in Figs. 4, 5, 6, and having 

 found from former experience that a yellowish 

 tinge would help by modifying the blue-green 

 of this screen, we add to it a second one of 

 picric acid, the holder being constructed to 

 carry several in combination when necessary. 



