THE MICROSCOPE. 125 



Swing the candle to the right until the field ofview, seen through 

 the microscope, is one-half illuminated, bisected vertically. 

 mark the position of the candle on the table, repeat the opera- 

 tion on the left side, marking the position there. A line drawn 

 from the centre pin to each marked point will show the angle 

 oi aperture, approximately, (usually a little too large), which 

 can be measured with an ordinary protractor. Microscopes hav- 

 ing a graduated circle on the mirror-bar, may have a wax taper 

 fastened in the place of the mirror, and the angle, obtained as 

 above, read from the circle. This angle of aperture bears a cer- 

 tain relation to N. A. The rule is, the sine of half the angle of ap- 

 erture— JV. A . for dry objectives. A table of numerical aper- 

 ture has been calculated, and is published in Carpenter, and 

 other works, also in several catalogues of optical instruments- 

 This table gives the N. A., for any given angle of aperture. 



Say, a dry I inch objective was measured as above and found 

 to include an angle of 110.° Reference to the numerical aperture 

 table will show that a dry objective, 110°=N. A. 82. Obtained by 

 the rule, one half of 110,°=55.° By reference to a table of natural 

 sineF, (found in works on trigonometry), the sine of 55°=82. 



167. How may I tell the thickness of a cover-glass by using the 

 micrometer on the fine adjustment of the microscope ? — B. E. R. 



Focus successively on the dust on the upper and lower sur- 

 faces of the cover-glass. For a mounted object hardened under 

 pressure, search for a point in the object which requires the 

 greatest elevation of the objective for precise focus. This point 

 is presumably in contact with the cover, and the difference be- 

 tween this point and thedirton the upper surface of ihe cover 

 may be measured by the movement of the divided screw-head. 

 The values of the divisions on the screw-head must be obtained 

 by trial over covers of known thickness. 



PRACTICAL SUGGESTIONS, 



By L. A. WILLSON, CLEVELAND, OHIO. 



How to Produce Pretty Crystals of Borax.— Make a sat- 

 urated aqueous solution of borax. Place a drop of this solution 

 on the center of the slide. Set the slide on a window-sill where 

 the draft will affect the slide. 1M the preparation stand until 



