30 THE MICROSCOPICAL NEWS. 
lens of 175°. Our comparison, therefore, may be confined to the 
low and medium powers. 
In 1855, Mr. Andrew Ross produced a new series of objectives, 
WEE 3 mene 15° and 22°; 4 in. of 65°; 7 in. of 85° and 125°; ¢ in. 
of 135°; 4in. of 130°and 150°. He had previously produced a one- 
inch of 27°. About this time Messrs. Smith and Beck produced 
a ‘5ths of go. The Micrographic Dictionary for 1860 tells us 
that the lenses then in use had the following angular apertures :— 
2 In. OF If in. 12°to 20°; 1 in. or Z in. 22° to 35°; 4 or yp In. 65°; 
¢ In. 75° to 140°; $ in. r10° to 150°. As far as I can ascertain, 
they represent the common apertures of the period alluded to in 
the Address. They are all made at the present day, and in some 
instances are much exceeded. 
Mi. Tolles, of Boston, U.S. A., now makes the following lenses : 
zo inch up to 145°, + inch up to 180°. He has recently made a 
% inch of 70°, which is said to resolve Pleurosigma angulatum 
without a condenser. Mr. Spencer, of Geneva, N. Y., makes a 
1 inch of 50°, } inch of 100°, and 3 inch of 180°. Dr. J. Edwards 
Smith, Professor of Histology at Cleveland, Ohio, in his work, 
“How to See with the Microscope,” recommends, for ordinary 
biological work, a one-inch or two-thirds objective of 45° or 50°, a 
one-sixth of 87° to 95° balsam-angle (N.A. 1.05 to 1.13), a one-tenth 
of 100° balsam-angle (N.A. 1.17), and a dry one-half inch of 38° for 
work over chemical re-agents. I think it can scarcely be said 
that this represents a phase of experience through which micros- 
copists have passed in this country. 
The Address tells us that “increased angle has given us great 
power of resolution, but what else? Nothing at all.” Against this 
I would place a statement made in the last edition of the Doctor’s 
work, “The Microscope and its Revelations,” in which he says, 
“It is clear that the representations of minute structure given by 
objectives of widest aperture are more trustworthy than those given 
by those of zarrower.” 
The Address also tells us that “the flagella of Monas termo 
would probably not have been found without the wide-angle lens, 
but now they are known to exist, they have been seen better with a 
lower angle.” Theory tells us that the reverse of this ought to be 
the case, and in Prof. P. Martin Duncan’s presidential address to 
the R.MLS. we find these words :—“ In searching through a stratum 
of fluid for bacteria a wide aperture would be unnecessary, but when 
a particular bacterium is found, it is only that which will give us 
an accurate view of its flagellum.” 
The Doctor is careful to depreciate wide apertures on account 
of the accompanying loss of penetration, and instances the “high 
biological work” of Mr. Dallinger and Dr. Drysdale as having 
been “accomplished with a dry z; inch of 140°. For a more cor- 
