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Transactions of the Society. 
mounted in a dense medium, stand out clearer and sharper than those 
which have been plumbagoed and mounted in balsam. The Society 
must he congratulated on possessing such a fine standard for micro- 
metrical measurements. 
A new form of camera has been introduced by Messrs. Swift and 
Son. Cameras may be divided broadly into two kinds : first, those 
suitable for a Microscope when used in a horizontal or nearly hori- 
zontal position ; secondly, those for use when the Microscope is in a 
vertical or nearly vertical position. In the first group we have those 
of Wollaston, Beale, and their modifications ; in the second, those of 
Abbe and several others. The Abbe form was invented by Mr. G. 
Burch in 1878, as an alternative arrangement for his micrometer.* 
(Let me again express my astonishment that this efficient, simple, and 
inexpensive arrangement of Burch’s micrometer is not supplied by 
our opticians. A device so eminently practical could not but com- 
mand a large sale among biologists and students.) 
Swift’s camera differs from that of Abbe in two points: first, 
instead of the oblique side of the prism being opaquely silvered, with 
a hole in it, in Swift’s it is very lightly silvered, so that it is semi- 
transparent and no hole is required ; secondly, the large plane mirror 
of the Abbe camera is replaced by a small right-angled prism, the 
hypotenuse of which is silvered. The advantage of the semi-trans- 
parent silvered film in Swift’s camera is, that it tones down the light 
in the Microscope, and no extra screen is required ; also the small 
prism is far lighter than the large plane mirror, all strain, therefore, 
at the eye-end of the body of the Microscope is removed. 
Messrs. Zeiss have exhibited here a new lens called a “ Plankton 
Searcher,” a lens suitable for the examination of wandering or 
roaming objects ( 7 r\a£co, to wander or roam). It is a low-power 
water-immersion of 1 ’3 in. focus and * 11 N.A. aperture. This lens is 
practically apochromatic, and its definition is exceedingly sharp and 
bright ; the illumination, owing to the greater refraction of the water, 
is greater than that of an ordinary dry lens of the same diameter in 
the proportion of 1:1 j. A good method of illuminating objects for 
examination with this lens is to focus, by means of a bull’s-eye, the 
image of the fiat of the flame of a paraffin lamp upon them through 
the side of the cylindrical glass vessel, by which means an excellent 
velvety dark ground can be obtained. Let me express a hope that 
this lens is only the first of a series, and that its success will encourage 
the firm of Messrs. Zeis3 to extend both the powers and apertures. 
The bottom of the glass vessel is optically worked ; it is therefore 
advisable to gum beneath it a circular edging of paper, to prevent it 
becoming scratched by contact with the metal stage. 
There is one other important departure during the past session 
to be noted, viz. the improvements that have been effected in cheap 
student’s Microscopes by Messrs. Beck and Watson. I can perfectly 
* Journ. Quekett Micr. Club, v. (1878) ser. i. p. 45. 
