THE SELECTION OF OBJECT-GLASSES. 81 



J and Jth, with a magnification ranging from about 125 

 to 250 diameters with the A eye-piece, and increasing 

 with the b eye-piece to 375 diameters. I have in 

 my possession a fine J made for me by Dallmeyer, 

 with an aperture of 120. It bears an extra deep 

 eye-piece, and will then give a magnification of 1,000 

 diameters in every way satisfactory. It also works 

 through almost any thickness of cover-glass ; its 

 aberrations being equally well balanced for uncovered 

 objects, no mean test of a good objective. These 

 several points prove that its working aperture has been 

 brought to the maximum of utility. On the whole, 

 the power is one of considerable value for the investi- 

 gation of organized structure and for viewing living 

 action. Every one aiming at original observations upon 

 the morphology of living creatures should become 

 skilled in the use of high magnifying powers, as the -J, 

 7*05 T 1 2 > "iV an ^ "sV- -^ nave > however, always pointed to 

 the futility of constructing higher power object-glasses, 

 except with a proportionately increased width of aper- 

 ture. As the maximum angle appears to be 180, or 

 160, for the odd 20 are not worth the having (compare 

 the chords of 180 and 160 there is hardly any differ- 

 ence), and as a T ^ can be made to transmit an angle of 

 160, I maintain that it will possess as much resolving 

 power as any dry -fa or -g^th. 1 This is seen in the series 

 of wonderful photographs of muscular tissue, blood 

 corpuscles, etc., taken by Dr. Woodward, of Washington, 

 U.S., and which certainly prove that the photographic 

 eye sees after making every allowance for what is 

 due to the nature and undulations of light what the 

 human eye cannot see. 



The Immersion System. About fifty years ago 

 Amici demonstrated the value of a drop of water in- 

 serted as an adjustable film between the object and 

 the objective, and showed that it materially assisted in 



(1) Professor Abbe has shown that no objective can possess at the same 

 lime penetrating power and perfect definition. The practical outcome of 

 this observation is, that neither penetrating objectives nor defining objectives 

 Rie aloi.e sufficient for every kind of microscopical work. Both are neces- 

 sary. If, however, the student is limited in the number of his objectives, 

 then he should at least provide himself with a low-power defining lens, 

 |-inch Ross, and high-power penetrating immersion. 



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