Royal Microscopical Society. 
227 
III. — Remarlcs on the Resolution of the Nineteenth Band of 
Nobert’s Plate by certain Objectives, especially by a new Tolles’s 
Immersion j-gth. By Dr. J. J. Woodward, U. S. Army. 
( Taken as read before the Royal Microscopical Society.) 
In the ‘Monthly Microscopical Journal’ for September, 1871 
(p. 150), I published a short note on the resolution of Amphipleura 
pellucida by a Tolles’s immersion ’Jh. In a subsequent note dated 
November 21, 1871, and published by the same Journal in January, 
1872 (p. 27), I mentioned that I bad engaged Mr. Tolies to con- 
struct a higher power objective for the Museum, which had not yet 
been received, and remarked, “ should it fulfil Mr. Tolles’s expecta- 
tions it will give me as much sincere pleasure as it can to him or 
any of his friends, and I will promptly make the facts public.” The 
objective finally supplied in accordance with this order was put into 
my hands in August of this year. Mr. Tolies, however, informs 
me that the optical combination was completed and tested by him in 
May. This objective is so surpassingly excellent on the plate, that 
I hasten to make the facts public, as promised. 
The objective is made to work wet or dry at pleasure. Wet, it 
is marked by the maker T Vth. The dry front at fifty inches dis- 
tance from micrometer to screen magnifies 800 diameters uncovered, 
900 covered. Angle of aperture 125° uncovered, 155° covered. 
The wet front can also be used dry for the first half turn of its 
screw-collar from the extreme open point, the range of powers and 
angles being very nearly the same as with the dry front. The 
next half turn of the screw-collar is intended for work wet, the 
maximum angle as measured in air being above 170°. An arrow 
on the tube of the objective, and a zero on the screw-collar, indicate 
the limit proposed for uncovered wet. At fifty inches distance the 
powers are 900 diameters when the collar is brought to the zero 
point, 1050 diameters when completely closed. On trial, however, 
I could not satisfy myself that the zero point was accurately marked. 
I first noticed that on several slides of diatoms, mounted with ex- 
tremely thin covers, the best definition wet was obtained when the 
objective was opened to the zero point, or even yet farther ; then 
testing the objective wet on a broken Nobert’s plate, mounted with 
the lines uppermost and uncovered, I found that the bands were 
best seen when the correction was one-sixth of a whole turn more 
open than the zero point marked. At this correction the magnify- 
ing power at fifty inches distance was 845 diameters. At ten inches 
by lamplight it was 170 diameters, the best correction on the naked 
plate by lamp being sensibly the same as by sunlight, or if anything 
a trifle more open. I made the latter determinations with the 
utmost care for the benefit of those who think the several fronts of 
