ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



399 



may be briefly indicated. In the first place, no advantage can result 

 from the use of the outer edges of the lines, instead of the middles, as 



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PP 



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is usually done, as the part of the lines from which distances, and 

 therefore N.A.'s, must be estimated by eye. Further, in Nelson's form 

 the thickness of the lines varies in different parts of the diagram, and 



