ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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time qnito a sound fitting. In brief tlio three forms are — 1st, tho rack 
let into the inner tube; 2nd, tho rack boxed in tho outer tube; 3rd, the 
rack passing through the collar.” 
Fig. 20. Fig. 21. 
(2) Eye-pieces and Objectives. 
Use of the Microscope with Hig;h-power Objectives.* — M. H. 
Peragallo points out, without entering into superfluous theoretical con- 
siderations, the method of using high-power objectives so as to obtain 
from them all possible advantages. He first gives a brief resume of the 
theoretical considerations concerning the formation of images in the 
Microscope. The classical theory of the formation of images in the 
Microscope, founded on the emission theory, has long been known to be 
incapable of furnishing a complete explanation of optic phenomena. 
Abbe was the first to apply to the formation of images an exact mathe- 
Ann. de Microgr., iv. (1892) pp. 585-616. 
