4 THE AMERICAN MONTHLY [Jan. 



thus equipped is tlie auxiliary microscope. 



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

 of the microscope. It is semicircular, with the semicir- 

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

 the back edge is ground at an angle of -lo", base of the 

 prism upwards. 



The upper surface has two sets of graduations on it, 

 the outer circle being for numerical and the inner fot 

 angular aperture. Corresponding to the centre of the 

 circle is the small perforated silvered disc, mounted under 

 a cover glass, and through which the linage of the indices 

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

 circle, slide two L shaped indices so made as to hang on 

 the upper edge of the disc and lie against the vertical 

 margin. The light horizontally striking the vertical edge 

 of the plate glass disc projects the images of the indices 

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

 horizontally along the diameter of the semicircle directly 

 under or to the right and left of the objective 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 perforated 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 

 barely visible within the circle of light. The reading is 

 then made direct from their inner edges in numerical or 

 angular aperture as desired. 



For the experiment herein detailed, a substage conden- 

 ser and iris diaphram are necessary accessories, although 

 one may proceed in a crude and unsatisfactory way with- 

 out the latter. 



The objectives whose apertures are to be compared, are 



