ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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beneatli the field lens which remains immovable in the axis, while the 
field and eye-lens move over it. The lower plane surface of the field 
lens has diagonal cross lines, as well as a double index-mark engraved 
on it, and the plane upper surface of the additional lens has a scale. 
The lens has a focal length of about 17 mm., so that its lower focal 
point lies approximately in the opening of the objective, and the prin- 
cipal rays are made parallel in front of the eye-piece as if their centre 
of divergence was at infinite distance. In consequence of this, in every 
position of the eye-piece, the point of the field under observation 
behaves as the centre of the field of view in the ordinary arrangement, 
that is, all the pencils are identical with the axial pencil, and the shifting 
of the eye-piece produces no optical excentricity.^ 
Winkel’s Combination of Screw-micrometer and Glass-micrometer 
Eye-piece. I — Dr. A. Koch writes as follows: — For fine microscopic 
measurements, in particular for the determination of the thickness of 
Bacteria, it appeared to me to be of advantage to possess an apparatus 
with which exact determinations could be made more easily than with 
the ordinary eye-piece micrometer. I found useful for this purpose an 
eye-piece with a thread such as has been in use for a long time in 
physical and astronomical instruments, and occasionally employed in 
Microscopes. In these eye-pieces a stretched thread or a mark on a 
glass plate can be moved parallel to itself by means of a micrometer- 
screw with divided head ; for measuring, the thread is brought succes- 
sively to both edges of the object, and its breadth is given by the number 
of turns of the micrometer-screw necessary to move the thread from one 
margin of the object to the other. The value of the divisions of the 
drum of the micrometer-screw is determined by an object-micrometer. 
It is, however, inconvenient, especially with very strong magnifi- 
cations and very small objects lying in great numbers in the field of 
view (such as Bacteria) to have to replace such a screw eye-piece by an 
ordinary micrometer eye-piece when it is desired to measure with less 
exactness the larger divisions of the object which we had previously 
been measuring with the screw eye-piece, e. g. the length of a Bacterium. 
Herr E. Winkel, of Gottingen, has, however, constructed a micrometer 
eye-piece in which the thread is replaced by a division on a glass- 
micrometer. This apparatus can therefore be used, as I have already 
mentioned in my work, ‘ Ueber Morphologie und Entwicklungsgeschichte 
einiger endosporer Bacterienformen,’J either as an ordinary micrometer 
eye-piece with fixed micrometer for less fine measurements, or for more 
exact determinations by using the micrometer-screw and successively 
adjusting one edge of a division on the margins of the object. 
The mechanical details of the apparatus are shown in figs. 45 and 46, 
the latter fig. showing the internal arrangement after the upper part at A 
(fig. 45) has been unscrewed. In fig. 46 is seen the frame DE, which 
is moved by the micrometer-screw EF, which has a pitch of exactly 
1/5 mm., and on which lies a glass micrometer divided in 
* Cf. Dippel’s ‘Handbuch der Allgememen Mikroskopie,’ 2nd ed., 1882, 
pp. 639-40 (2 figs.). 
t Zeitsclir. f. Wiss. Mikr., vi. (1889) pp. 33-5 (2 figs.). X Bot, Ztg., 1888. 
