THE VALUE OF EYE-PIECES. 



57 



dissected insects, the vascular and cellular tissues of 

 plants, the Haversian canals, the lacunas of bone, and 

 the serrated laminaa of the crystalline lens of the eyes 

 of birds and fishes require the B eye-piece. 



The C eye-piece is brought into use when it is neces- 

 sary to investigate the structure of very delicate tissues ; 

 and in observations upon minute diatoms and desmids, 

 scales of moths, gnats, raphides, etc. The employment 

 of a deep eye-piece sometimes obviates the necessity of 

 using a deeper object-glass, and which always occasions 

 a re-arrangement of the illumination generally. It 

 must be borne in mind, that the more powerful the 

 eye-piece, the more palpable will the imperfections of 

 the object-glass become ; hence less confidence should 

 be placed in observations made with a powerful eye- 

 piece than when amplification is obtained with a 

 shallow one and a deeper object-glass. 1 



The Draw-tiibe is an intermediate tube, which when 



(1) AMPLIFICATION OF OBJECTIVES AND EYE-PIECES. 



Reduced from a comprehensive table of the magnifying power of eye- 

 pieces, and the amplification of objectives and eye-pieces combined, issued 

 in a separate form with the Journal of the Royal Microscopical Society. 



