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A concave mirror or lenticular prism can also be used for the
same purpose, provided the focal length is adapted to the length
of the body and object-glasses.
Various modifications can be adapted so as to vary the character
of the illumination to suit the particular object to be viewed. In
some case^ the Wenharn's prism may be withdrawn a little, to
produce the proper effect.
Uncovered objects only can be seen to advantage, owing to the
light reflected from the surface of the covering glass. The surface
on which the objects are mounted should reflect as little as possible, and be a marked contrast in colour to the object.
Ordinary Meeting, November 28£&, 1865.
R. ANGUS SMITH, Ph.D., F.E.S., &c, President, in the Chair.
Mr. Francis Hainpson, solicitor, was elected an OrdinaryMember of the Society.
Mr. Dancer, F.R-A.S., said that in a paper " On the Illuminaof Opaque Objects under the High Powers of the Microscope,"
read before the Microscopical Section of this Society, November
20th, he had described a method of employing the oblique body of
the binocular microscope with Wenham's prism, for illumination
of opaque objects, and he had also exhibited an instrument fitted
up for this purpose, giving the members present a practical
demonstration of the advantages which this mode of illumination
afforded under certain circumstances. He wished now to describe
another method of illuminating opaque objects, and, as it is equally
applicable to monocular and binocular microscopes, it appears
worthy of some consideration.
In the method of Mr. H. L. Smith, of Kenyon College (which
was briefly described in the paper before mentioned), and also in
the use of the "Wenham's prism, there is a considerable loss of
angular aperture (which is a very important consideration). It
occurred to the author that by modifying Mr. Smith's contrivance
this loss might be diminished in some degree; this has been
attempted in the following manner.
Instead of placing the mirror immediately over the opening
at the back of the object-glass, a small speculum £th of an inch in
diameter is introduced into the front of the body of the microscope, 2£ inches above the top of the objective. A lateral opening
is made in the body at right angles to the speculum, for the admission of light to be reflected down through the objective to the
object below.
The interposition of the small speculum does not produce any
disagreeable effect in the field of view, and in the examination of
objects it is easy to use that portion of the field which is between
the centre and the edge. With proper manipulation very good
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definition can be obtained by this method when the speculum is
of the proper curvature. This contrivance can always remain
attached to the microscope without interfering with the general
appearance of the instrument, and when the use of the speculum
is not required it can be withdrawn or turned aside out of the
field of view, and the aperture at the side of the body may be
closed by a small shutter. It is obvious that the use of the
binocular body is not interfered with by this arrangement.
A binocular and a monocular microscope with this arrangement were exhibited to the members at the close of the meeting.
December 18th, 1865.
J. B. DANCER, F.H.A.S., in the Chair.
Mr. Parry exhibited some sections of fossil wood and Echinus
spines, most beautifully cut by Mr. John Butterworth, of Oldham,
and presented some of the slides to the Section.
Mr. Parry also presented to the meeting, for distribution among
the members, mounted slides of the contents of a shark's stomach,
from the Madras coast, consisting almost entirely of Diatomacese.
Mr. Hurst then made a few remarks on late improvements in
illuminating opaque objects under the higher powers of the microscope. He said they consisted of three different methods. Firstly
that of H. E. Smith, of Kenyon College, America, described in
the English ' Mechanics' Magazine' of the 20th October, 1865,
in an extract from the 'American Journal of Science and Arts.'
This gentleman employed a box, or adaptor, between the objectglass and the "Wenham's prism of the binocular, with a side
perforation, opposite to which was a small silver reflector or a
common thin glass cover, acting as a mirror and capable of adjustment to any angle, thus enabling it to throw the rays of light
admitted by the side aperture through the object-glass down on
to the object itself.
The disadvantage of this method is that all adaptors cause
unsteadiness, and, however skilfully constructed, injure the accurate centering of the object-glass; and while, on the one hand, the
thin glass cover appears to produce some distortion of the image,
the reflector so near the object necessarily casts off a number of
the rays proceeding from it. This plan also seems to require
lamp-light and the use of a condenser. Messrs. Smith and Beck
appear to have patented the use of the thin glass cover.
Secondly, a modification of the foregoing by Mr. Dancer, of
this Section, who places the thin glass or reflector between the
eye-piece and the Wenham prism, cutting an aperture in the
body of the microscope to admit the light. This dispenses with
the objection inherent to adapters, and theoretically seems the
most perfect of these new methods ; but Mr. Hurst's experience