4 THE AMERICAN MONTHLY [Jan. 



tlins equipped is the anxiliary mici(»scu[ie. 



The disc (b) is of phite glass and is phieed on the stage 

 of the inicroseope. It is seniicireuK-ii-. with tli^^ semicir- 

 cular niaigin vertical and polished, as are all its surfaces; 

 the hack edge is ground at an angle of 4n°, base of the 

 prism upwards. 



The uj)})er surface has two sets of graduations ou it, 

 tlie outer circle being for numerical and the inner for 

 angular aperture. Corresponding to the centre of the 

 circle is the small perforated silvered disc, mounted under 

 a cover glass, and through whicli the iinacie of the indices 

 is observed. Over the right-angled margin of the semi- 

 circle, slide two L siiaped indices so made as to hang ou 

 the upper edge of the disc and lie against the vertical 

 margin. The light horizontally striking the verti<-al edge 

 of the plate glass disc projects the images of the indices 

 on the margin in such a manner that they ajjpear to lie 

 horizontally along the diameter of the semicircle directly 

 under or to the right and left of the (dtjective according 

 as they are moved. 



The indices are brought near the centre of the margin 

 of the semicircle, and by sliding the draw tube up or 

 down within the body tube (care being taken not to alter 

 the focus of the objective to be measured which has been 

 focussed on the centre of the silvered i>erforated disc 

 previous to attachment of auxiliary objective to draw 

 tube) a sharp image is obtained of the indices. They are 

 then moved around one on each side, until their points are 

 V)arely visible witliin the circle of light. The reading is 

 then made ilirect fi'om their inner edges in nunierical or 

 angular aperture as desired. 



For the experiment liiTfin detailed, a substage conden- 

 ser and iris diaphram are necessary accessories, although 

 one may proceed in a ciikIh and unsatisfactory way with- 

 out the latter. 



The objectives whose apertures are to lie compared, are 



