PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 



779 



without the focus. The coloring is now termed nonsymmetrical because the edges arc 

 not colored the same. 



The reasons for this nonsymmetrical coloring with oblique light are illustrated in 

 Fig. 11. Light enters from the lower third of the substage condenser and illuminates 

 the white space of the test plate A^-A^. At A^ the image is really in the apple green, 

 but, unless very closely observed, this color is lost because of the brilliancy of the white 

 space. The orange-blue (omitted to simplify the diagram) and the blue-violet appear 

 as passing through the black space and have been so represented in the diagram. At 

 A^ just the opposite conditions obtain. The orange-blue and the blue- violet are lost 

 and the yellow-green appears to come through the black space. No matter whether 

 the objective is raised or lowered, the same color relationship persists. 



Ye/Zo 



pv green 



~Blue vfolef 



Fig. 10. 



Whether the objec+ive is wi+hin or 

 wi+hou+ the focus +he colorinoj 

 remains +he same 

 -Coloring of borders of subject when oblique light is used with test plate and 

 semiapochromatic objective. 



Fig. 11. — Diagram illustrating nonsymmetrical coloring of the type shown in Fig. 10. 



Cleaning Objectives and Oculars. — Objectives must be clean if they are to perform at 

 their best. After an immersion objective is used, the immersion fluid should be 

 wiped from the front lens with a fresh piece of lens paper moistened with pure ben- 

 zene. It may be necessary to use several pieces of lens paper and repeat the opera- 

 tion until the lens and its mount are clean, bright, and dry. The objective should 

 then be stored in its case. When handling objectives, the fingers should be confined to 

 the knurled collar, and they should not come in contact with the front and back lenses. 



To care properly for objectives and oculars simple appliances are useful. A supplj' 

 of very clean fine-grade absorbent cotton should be stored dust free in a covered glass 

 vessel. Tufts of this cotton are twisted securely about the ends of wooden applica- 

 tors. Care should be exercised to see that the end of the stick is deeply buried in the 

 cotton. This cotton swab is then moistened in benzene and the lens surfaces gently 



