ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
119 
represents tlie apochromatic 2 mm. aperture 1*40, showing the front 
lens held by the lower ridge only ; while the right side shows the 
apochromatic 2 mm. aperture 1 • 30 with the much wider setting. 
As regards the durability of the apochromatic lenses, the firm state 
that all glasses used in their construction having been amply tested 
through several years’ experience, offer a fair guarantee for their per- 
manency, at any rate in temperate climates. In tropical climates their 
use is not advised. In damp or hot climates they should not be kept 
in closed receptacles, as they are liable to be affected by stagnant damp 
air. 
The apochromatic 2 * 5 mm , aperture 1 * 60 (monobromide of naphtha- 
lene immersion) has hitherto been used successfully for the examination 
of Diatomaceae only, since no mounting medium of sufficiently high 
refractive index, 'which can be used for other organisms without destroying 
their structure or colour, has yet been found. 
Zeiss’ Screw Micrometer Eye-piece.* — In this instrument for exact 
measurements, which is shown in sectional elevation and plan in tig. 13, 
the glass plate with crossed lines is moved across the field by means 
of the micrometer-screw. The instrument is provided either with a 
Fig. 13. 
Eamsden or with a compensating eye-piece, and is clamped to the upper 
end of the body-tube by means of the screw shown on the left side of 
the figure. Each division of the divided drum corresponds to 0*01 mm. 
of the objective image. 
Zeiss’ Eye-piece for Observing Axial Images.! — This eye-piece for 
axial images (fig. 14) consists of a Huyghenian eye-piece No. 2 with 
* Zeiss’ Catalogue, No. 30, 1895, pp. 74-5. 
t Op. cit., p. 8G. 
