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merely a question of angular aperture. Ross's -jVth, Smith’s 
-^th, Powell and Lealand’s -jVth, and -^th dry objectives 
were all of larger aperture than Merz’s T ‘ ff th Immersion , 
yet neither gave as good results on Surirella gemma, Frus- 
tulia Saxonica, Grammataphora Subtilissima, P. Angulatum, 
P. Attenuatum, P. Macrum, &c., nor on Nobert’s plate, as 
Merz’s objective. Hartnack asks the question, “ Are large 
apertures an advantage to the microscopist or to the optician ?” 
and he frankly says that “ the disadvantages are for the 
latter only.” 
The advantages mainly claimed for the Immersion ob- 
jectives are : — greater working distance between the object 
aud objective, increase of light and superior definition and 
clearness in the optical image, which image is obtained by 
much simpler illuminating apparatus and with less manipu- 
lative skill than that considered indispensable in using high- 
power dry objectives. 
It is not difficult to see that Amici’s system, of connect- 
ing the objective with the cover-glass by a film of water, 
very much diminishes the reflexion which necessarily takes 
place on the incidence of oblique light when the dry objec- 
tive is employed. The limiting angle of refraction in water 
being about forty-eight degrees, it follows that, whatever is 
the degree of obliquity in the incident light on the object, 
the Immersion objective never has to do with rays of greater 
obliquity than forty-eight degrees. To this, in great measure, 
is due the greater clearness and precision of image obtained. 
Continental opticians and men of science have been aware 
of the merits of the Immersion system during several years 
past ; and to such purpose, that knowing how little atten- 
tion it has received here, they do not scruple to say that the 
English no longer take the lead either as opticians or as 
microscopists. 
There are amateurs who make a speciality of declaring 
that all Test-objects can be resolved by some wonderful pth 
objective. I am told, for instance, that coarse transverse 
and very fine straight longitudinal lines can easily be seen 
with a |th on Surirella gemma. It is well known that a 
low power will show transverse lines ; but no objective or 
method that I knoAV of shows straight longitudinal lines on 
this diatom. The finest images that I have seen of Suri- 
rella gemma (with Hartnack’s -j^th and Nobert’s -<Ath), 
do not confirm Hartnack’s opinion that the surface is a 
series of elongated hexagons, similar to the drawing in his 
pamphlet on “ Test-objects” (published in 1865, and copied 
by Dr. Carpenter in the last edition of his work on “ The 
