ON THE HIGHEST MAGNIFYING POWEES. 
421 
glasses. The third lens is single, while in the objective of the 
last makers it is composed of three. The recently made 
by Messrs. Smith & Beck was also made upon this plan. 
The method adopted for correction for the varying thickness 
of the thin glass is also different in Mr. Wenham’s object- 
glass. The two upper of the three glasses of which the lens 
consists are movable instead of the lowest one, which is fixed ; 
by which excellent arrangement the correction may be made, 
while the position of the third lens is unaltered : so that 
one may watch the specimen while correcting for the thin 
glass covering the object. This plan is so far superior for 
practical working to that in ordinary use, that I am surprised 
it is not invariably adopted in the case of all very high 
powers. The definition of Mr. Wenham^s is excellent, 
and for the examination of such objects as the navicula 
I think it possesses advantages. 
I now pass on to the consideration of Powell & Lealand^s 
new glass, the fiftieth. This glass defines even better than 
the twenty-fifth. Plenty of light for illuminating the objects 
to be examined is obtained by the use of a condenser provided 
with a thin cap, having an opening not more than the 
of an inch in diameter. The angular aperture of this glass is 
not more than 150®, but as is well known many twelfths have 
been made with a much higher angle = 170°. The preparation 
may be covered with the thinnest glass made by Messrs. 
Chance, of Birmingham, or by mica, and there is plenty of 
room for focussing to the lower surface of thin specimens, 
which can alone be examined by high powers as transparent 
objects. 
I would draw attention to these very high powers at this 
time more particularly, because the facts recently urged in 
favour of the doctrine of spontaneous generation, lately re- 
vived, may be studied with great advantage. Not only are 
particles, too small to be discerned by a sixteenth, well seen 
by a twenty-fifth or a fiftieth, but particles too transparent to 
be observed by the twenty-fifth are distinctly demonstrated 
by the fiftieth. I feel sure that the further careful study, by 
the aid of these high powers, of the development and increase 
of some of the lowest organisms, and of the movements which 
have been seen to occur in connexion with various forms of 
living matter (Amoeba, white blood-corpuscle, young epithelial 
cells, &c.), will lead to most valuable results bearing upon* the 
much debated question of vital actions. 
Another very great advantage resulting from the use of the 
highest powers occurs in minute investigations upon delicate 
structures which occupy different planes, as is the case in mau}^ 
nervous organs. In studying the distribution of the nerves 
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