THE MICROSCOPE. 
Vou. VI. DETROIT, NOVEMBER 1886. No. 11. 
ORIGINAL COMMUNICATIONS. 
A CENTRAL LIGHT OBJECTIVE. 
A. Y. MOORE, MICROSCOPIST. 
FEW days ago H. R. Spencer & Co., sent me a new objective 
which was made for Dr. C. Mitchell, of Nashville, Tenn., 
with the request that I should thoroughly test it. This I have 
done, and, as the results may be of interest to many I should like 
to give them. 
The objective is marked ‘1-18—105° B. A.,” und is the first 
objective of that power constructed for the biological series, which 
has lately been introduced by Spencer. The lens is “ first-class ” 
and is mounted in the first-class adjustable mounting with gold 
plated front, as is usual with Spencer’s first-class objectives. 
Measured in Chancie’s hard crown glass, the aperture is 107° 
with the systems closed and 96° when opened to the full extent. 
The magnifying power, measured at ten inches from the object— 
which, in such a high power is practically ten inches from the op- 
tical center, is at closed, 200 diameters, while by opening the sys- 
tems the power is reduced to 162—-thus making an average of one. 
eighteenth inch focus. The working distance, at closed, is one- 
fifty-fifth (0.0184) inch and at open point one-sixty-sixth (0.0152) 
inch. As stated in their price list, the objectives of the biological 
series are constructed and corrected with special regard to central 
light work with homogeneous immersion fluid, and as a natural re- 
sult, this objective is over-corrected for color when used by oblique 
light or with any immersion fluid having a greater dispersion than 
that which produces the best results (chromatically) by centra 
light. Used with water the color is over-corrected, slightly, with 
either central or oblique light. ) 
The resolving power of this objective is better than that of any 
