204 



THE AMERICAN MONTHLY 



[November 



■1 



is large and linn, the simpler form 

 having a black glass top with slid- 

 ing objcct-cariier. All the stages 

 rotate, and are furnished with a 

 means of centering by lift and press 

 screws. All are provided with a 

 slip for Maltwood finder. The me- 

 chanical stage furnished with this 

 stand has motion of about ^ of an 

 inch at right angles, hy rack and 

 pinion and quick-thread screw, and 

 is also provided with a black glass 

 top. The stages are thin, admitting 

 the use of very oblique light. For 

 light of still greater obliquity, the 

 stage may be turned over so as to 

 have the object slide on the lower 

 slide of the stage. The fitting on 

 which the stage revei-ses is graduated 

 in single degi-ees for twenty de- 

 grees upon eitlier side for experi- 

 ments in resolution of lined objects. 

 This is accomplished without throw- 

 ing the object out of the center of 

 the swinging substage. The sub- 

 stage swings on a circle of 3|^ in- 

 ches diameter, and is graduated to 

 degrees ; it moves along the swing- 

 ing bar by rack and pinion. The 

 sub-stage may be centered in the 

 optic axis l)y two milled heads. This 

 centering arrangement is of new 

 construction, and is contained in 

 the space l)etween the substage 

 ring and the slide, thus doing away 

 with the clumsy ring and set-scrcM's 

 found on some of the substages by 

 other makers. 



The foot is a large tripod filled 

 with lead for M^eight, and may be 

 removed from the pillar, or may be 

 turned in any position in reference 

 to the stand. 



The mirror is large, and is both 

 plane and concave. It is so mounted 

 that the entire mirror and frame 

 may be removed, and a toy M^ax- 

 candle (in holder) inserted for mea,s- 

 uring angle of lenses. 



The engraver has not done justice 

 to the stand. The graduation on 



the circle has been omitted, and 

 the connection between the sul>- 

 stage and the bar is not shown. 



A Catoptric Immersion 

 Illuminator.* 



BY .JOHN WAKE STEPHENSON, F. K. A. S., 

 TKEA8. K. M. S. 



As the subject of immersion 

 illuminators is now before the So- 

 ciety (and I am very glad it is 

 so, for without their help the full 

 resolving powers of the recent 

 large-angled objectives cannot be 

 utilized), it may not be out of place 

 to lay before the Fellows a brief 

 account of an immersion condenser 

 of very simple construction which 

 T devised in 18TT. 



The diagram shows the form and 

 size of the instrument which I now 

 use, although it is sufficiently ob- 

 vious that other sizes, in the same 

 ratios, may easily be made — in fact, 

 I have such. 



The apparatus is simply a plano- 

 convex lens of flint glass, worked 

 on a one-inch tool, and having a 

 diameter of 1-2 inches, which is 



Fig. 30. 



then " edged " down to one inch, as 

 being more convenient in size, and 

 as giving an aperture sufficient for 

 our purpose. 



The upper or convex side of 

 the lens is cut down or flattened, so 

 38 to gives a surface y*^ of an inch 

 in diameter, with which the slide is 

 to be connected, when in use, by a 

 drop of oil or water. 



It matters not which fluid is 

 used as long as the objective has a 

 numerical aperture not exceeding 



* ^our. Roy. Mic. Soc. 



