Z00L03Y AND BOTANY, MICROSCOPY, ETC. 
687 
pactness being maintained at the cost of some unnecessary inconveniences. 
The possibilities of the English-American stand seem to be not yet 
realized on the Continent ; but the writer will venture the prediction 
that there will be a revolution in this respect some time before the next 
tricentennial. The reaction has evidently commenced already in respect 
of accessories and incidental refinements of detail. Such microscopical 
aids and comforts as mechanical stages, elaborate substages and sub- 
stage condensers, iris diaphragms and rapid nose-pieces, which we have 
been using for a generation, more or less, with great satisfaction, but 
which we have meanwhile heard constantly denounced by partisans of 
the Continental method as needless luxuries and distracting toys, all 
these we now see introduced and given a proper prominence by the best 
Continental makers. ’Tis well. 
As to objectives, the progress of the present day evidently centres 
around or stands in comparison with, the apochromatic system. Of the 
success of the system there is no longer a reasonable doubt, either as a 
scientific or as a commercial question. Characterized by the employment 
of new varieties of glass, of extraordinary optical properties, manufac- 
tured at Jena, and by the substitution of fluorite (natural fluor spar) for 
crown-glass in several of the lenses, it corrected spherical and chromatic 
aberration to an extent not before attained. Brought into existence by 
the researches and experiments of Abbe and Schott, and first successfully 
introduced at the Zeiss factory, in 1884, it is now adopted for their 
highest grades of objectives by all the above-named makers, except 
perhaps Watson ; and new series or varieties are being constantly 
introduced.* The resolution of Amphipleura pellucida in dots, which has 
been ably disputed and may be still doubted by some competent judges, 
is well within the limit of their capacity. The writer was fortunate in 
being afforded the rare privilege of witnessing the! official trial of the 
objectives by the jury, at which time the Zeiss 1/10 (2-5 mm.) of N.A. 
1 • 63, with monochromatic sunlight illumination, showed the dots (or 
* beads’) with beautiful distinctness and with perfect ease. They were 
seen at a glance over a large part of the shell at once, always alike and 
with remarkable freedom from any suspicion of uncertainty ; and when 
lost by change of focus or even by carrying the instrument to another 
table and readjusting the light, they could be recognized instantly, and 
every time, by simply correcting the fine-adjustment. As the most oblique 
pencils able to be utilized by this objective, as employed, ought, 
according to the Helmholtz theory, to be able to resolve lines of about 
6000 to the millimetre, and as the A. pellucida had about 3700 rows 
of the dots transversely and 5000 rows longitudinally, to the milli- 
metre, this resolution, while near enough to the theoretical capacity 
of the lens to give an impressive demonstration of a near approach to 
perfection in the construction, yet does not, by exceeding its theoretical 
capacity, throw discredit upon either the genuineness of the resolution 
or the accuracy of the optical theories involved. 
But the durability of the apochromatics is, most unfortunately, not 
equally plain. It is said that those first made were unsafe, from the 
* Near the close of the exposition, after the report on awards was adopted and 
closed, a new I/I 2 (2 mm.) was received from Nachet and informally examined by 
the jury with the result of calling out from them a special note of commendation. 
