ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
389 
to their lateral distance from the optic axis. Harting found that the 
image of ten divisions of a glass micrometer gave a result 1/33 less 
when it was measured as a whole than when the single divisions were 
measured and added together. The difference in magnifying power of 
the different parts of the field of view can of course be determined, but 
the process would be very tedious, and is not practically available. 
Dr. H. V. Mohl * therefore adopted the plan of moving the optical 
part of the eye-piece (with crossed threads) across the field. The imago 
Fig. 42. 
is thus always observed only through the axis of the lenses, and the 
distortion found in the case of the ordinary micrometer is avoided. As 
will be seen in fig. 42, the tube containing the eye-lens and field-lens is 
attached to a slide which is moved by the screw so that the axis of the 
lenses may be displaced laterally in relation to the optic axis of the 
Microscope, or in other words, the eye and field lenses can be made to 
traverse the field of view, f 
This micrometer, though it obviated the distortion caused by obser- 
vation through the excentric parts of the eye-piece, gave rise to a somc- 
* Arch. f. Mikr. Anat., i. (1865) pp. 79-100. 
t Fig. 42 does not represent Mohl’s micrometer as described by liim (of wliicli, 
indeed, we believe no figure is extant), but is taken from one sent us by Messrs. 
Merz, the makers of his original form. One important difference is that in the 
latter the eye-piece is not attached directly to the slide moved by the screw, but 
to a second upper slide which can be moved on the first by hand. The object of this 
is to adjust the lenses in the optic axis at the commencement of an observation 
without having to use the screw for that purpose. (We should be glad to be 
referred to a drawing or photograph of Mold’s original form if it exists.) 
The following is a condensed abstract of Dr. Mohl’s original description : — 
“As regards the mechanical details of the instrument constructed for Mohl by 
Steinheil, the Microscope-tube is screwed into a horizontal plate fixed on a solid 
standard, and carries a Fraunhofer screw-micrometer which works in agate bearings; 
above the micrometer is an orthoscopic Kellner eye-piece with a short tube. The 
eye-piece tube is not fixed directly to the micrometer-slide which is moved by the 
screw, but to a second slide which moves between swallow-tail guides upon the 
upper surface of the first in a direction parallel to the length of the micrometer- 
screw; this slide itself is moved by a second screw of deep pitoJi. The stage and 
condenser are separated from the boely of the Microscope, being carried by a bar 
wliich can be fixed to the stand of the instrument by means of two short arms. 
The eye-piece therefore is movable not only by the micrometer-screw together with 
the slide wh ich is used for measuring purposes, but also when desired by the 
second slide (or “ eye-piece slide”) which moves horizontally upon the first, so that 
