118 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
fine-adjustment milled head is divided, and has a pointer for recording 
the depth through which the focusing has been made. The draw-tube 
is graduated in the mm. scale, and shows exactly how much mechanical 
tube-length is in use (the tube-length being reckoned from the top of 
the draw-tube to the bottom of the nose-piece of the instrument). The 
base is perfectly solid, standing upon three flat points. The instru- 
ment being attached to a single pillar, allows plenty of room for work- 
ing the substage adjustments. 
Fig. 11 represents another of Messrs. Beck’s large Continental 
model Microscopes, but has different stage and substage adjustments. 
The large square stage is furnished with a vulcanite plate, and when 
the stage clips are removed, allows of a very large object or large 
culture plate being examined. The substage is arranged on a swinging 
arm, with a spiral focusing adjustment, and will carry any of the 
regular sized substage condensers. The construction of this instrument 
is almost identical with that of the other. 
(2) Eye-pieces and Objectives. 
Zeiss’ Apochromatics.* — The well-known special characteristics of 
these objectives, wdiich distinguish them from all other microscopic 
lenses, are : (1) the union of three different colours of tbe spectrum in 
one point of the axis, i. e. the elimination of the so-called secondary 
spectrum ; and (2) the correction of spherical aberration for two different 
colours, in contradistinction to the usual correction for one colour only 
in the brightest part of the spectrum. 
The apochromatic lens 2 mm. of 1*40 mm. aperture, owing to the 
hyper-hemispherical form of the front lens, which is supported by a very 
Fig. 12. 
narrow ridge at the edge of the setting, demands careful treatment. For 
this reason, although it possesses a greater resolving and defining power 
and gives a brighter image than the apochromatic 2 mm. of 1*30 
aperture, for regular work preference should be given to the latter in 
which the front lens has a much firmer setting. In fig. 12 the left side 
* Zeiss’s Catalogue, No. 30, 1895, pp. 8-13. 
