ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 



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light stopped off. Fig. 25 represents an oil-globule, a with the focus 

 on the margin, b somewhat higher, and c at the focal plane of the bubble. 



Oil-globules under the Microscope. Focus, a on the margin, 6 somewhat higher, 

 c higher (at the focal plane). 



Solid fibres, fig. 26, in a medium of lower refractive index (as a 

 glass thread in air or water) show a diffused moderately bright appearance 

 A at medium focus ; a bright 



central line B when the tube is Fig. 26. 



raised ; and a dull appearance C 

 when the tube is focused below 

 the centre. The reverse of course 

 takes place if the surrounding 

 medium is of higher refractive 

 index, as glass threads in mono- 

 bromide of naphthalin or binio- 

 dide of mercury and potassium. 

 If, again, the fibre is surrounded 

 by a fluid of about the same re- 

 fractive power, as in the case of a 

 glass thread in Canada balsam, it 

 will then have the appearance of 

 a flat band. 



Hollow fibres charged with 

 air, fig. 27 (or a fine capillary 

 tube of glass), present with medium focus nearly the same appearance as 

 the solid fibres, from which they are only to be distinguished by the fact 

 that at both edges the double outline of the section of their solid walls 

 will be seen as in A. In other respects the appearances are reversed ; 

 the raising of the objective giving a dull image C, whilst on sinking it 

 we have the central bright line B. Fine tubes in a denser substance 

 produce the same effect as hollow fibres. Semi-cylindrical channels or 

 furrows act as concave lenses, whether the hollow side is turned from or 

 to the observer. The only distinction between the two positions is, that 

 in the former case the tube must be focused lower than in the latter, in 

 order to obtain the greatest degree of brilliancy in the central line. 



If instead of the hollow fibre, or capillary tube charged with air, one 

 filled with a fluid is substituted, this produces the same effect as a solid 

 fibre, provided the contained and the surrounding fluid are nearly the 

 same, or if the former has a greater refractive power. Solid and hollow 

 fibres can then only be distinguished from each other in the medium 

 focus, showing the optical section of the solid walls. On filling with a 



Glass threads. Focus, A medium, 

 B higher, C lower. 



