1 886.] NEW- YORK MICROSCOPICAL SOCIETY. 29 



square opening which embraces the brightest part of the flame. 

 The hollow lens contains a dilute solution of ammonio-sulphate 

 of copper, prepared as follows : The lens is nearly filled with 

 distilled water, and then a strong solution of the ammonio-sul- 

 phate of copper is added till a turquoise color is obtained. The 

 solution is now cloudy from a partial precipitation of the hydrated 

 oxide of copper. Ammonia is now added until the precipitate 

 is just redissolved and the solution is free from any tint of green. 

 The depth of color of the fluid in the lens has to be adjusted by 

 trial, so that the blue of the lens shall just balance the orange of 

 the flame. Then a soft, white light appears in the field. This 

 intense whiteness, like alabaster, is very apparent in the dry 

 mounts of shells of Difilugiae, and frustules of Arachnoidisci, 

 exhibited before the Society. 



The plane mirrors, as generally made, of nearly all micro- 

 scopes, except those of the grand-models, are too small in the 

 front-and-rear diameter to illuminate the lower lens of dark- 

 ground illuminators. T obviate this defect by cutting an ellipse 

 out of a piece of plane mirror, and attaching this to the frame 

 of any plane mirror by means of four small pieces of cork ce- 

 mented to the back of the elliptical mirror. These pieces of cork 

 fit outside of the circular brass frame of the ordinary plane mir- 

 ror. The ellipse of the plane mirror has a minor axis a little 

 larger than the diameter of the lower lens of the illuminator. 

 The major axis is so long that when the mirror is inclined as 

 much as it will ever need to be, to the axis of the microscope, 

 the whole of the surface of the lower lens of the illuminator is 

 covered by reflected light. The shorter axis of the elliptical mir- 

 ror which I use is i^ inches, the longer axis is 2^ inches. 



The optical combination forming this illuminator is as fol- 

 lows : — 



There are three plano-convex lenses in contact with one 

 another. These may be designated A, B, and C, in their order 

 from below upward. 



A IS a. plano-convex lens with its plane side facing the mirror. 

 The radius of its curvature is 2^ inches. Its thickness through 

 its axis is TVTnrths of an inch. ^ is a plano-convex, with its con- 

 vex side down. Its radius is i inch. Its thickness is xTfths of 

 an inch. C is a plano-convex, of the same radius and thickness 

 as B. Its convex side is down. 



