ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
339 
Fig. 89. 
can be obtained without having the front of the condenser in immersion 
contact with the under surface of the slide. It has an aplanatic aperture 
of 0*90 N.A., that is to say, the whole of the light included in 0*90 N.A. 
is brought accurately to a point. The front lens is removable to make 
a low-power condenser. In the mount (fig. 87) the optical portion may 
be used as shown, or may be unscrewed and screwed into the bracket 
provided for the purpose at the bottom of the mount, and the whole 
condenser reversed. An iris diaphragm and a swinging arm with 
rotating fitting for coloured glasses or stops are provided. 
The condenser may also be supplied at a cheaper rate in a plain 
mount with iris diaphragm, as shown in fig. 88. 
Beck’s Hew Triple Hose-piece. — This is shown in fig. 89. Its 
great advantage is its dust-excluding properties, and it is also low- 
priced. The material throughout is bright lac- 
quered brass, and the nose-piece is accurately 
centered, and is so constructed that no dust can 
enter the back of object-glasses when left in 
position continuously on the Microscope. 
The Rational Use of Dark-Ground Illumi- 
nation.* — Dr. Gebhardt, in this paper, points 
out the neglect by scientists of dark-ground 
illumination, although he admits that botanists 
not infrequently use it. He thinks that this neglect is partly /lue to 
the incompleteness of the methods hitherto in vogue, notably to the 
deficiency of application of objectives of moderately large aperture. 
He points out that it is now possible, on the basis of the means fur- 
nished by Abbe’s illuminating apparatus, almost even without further 
aids, to render objectives of high aperture serviceable to dark-ground 
illumination. The method depends principally on the fitting of suitable 
diaphragms to the objectives. But, without special occasion, no stop- 
ping-off of dry objectives ought to occur, while with immersions it is 
always necessary. 
After numerous experiments with Zeiss’ apochromatics, it was ascer- 
tained that the optimum image was got for the aperture 0*30 with a 
stopping-off of about 0*10 to 0*15; for aperture 0*65 with about 
0 • 20-30 ; and that these afforded a sufficient penetration and flatness of 
the field, without the aperture becoming too much reduced for most pur- 
poses which require this flatness. With the aperture 0 * 95 two different 
diaphragms proved the best, one of about 0*30 and another of 0*60-70, 
according to whether it was desired to lay more stress on the flatness 
of the field or on the aperture. With achromatics similar proportions 
were obtained. With all objectives a considerable gain in light results 
from the water (or better the oil) immersion connection of the upper 
condenser-lens with the under side of the object-slide. With strong 
objectives of 0*70 upwards this is unattainable, if the full opening ought 
to be used, and in working with them the uniform use of a very intense 
light (e.g. sunlight) is advisel.j* 
* Zeitschr. f. wiss. Mikr., xv. (1899) pp. 289-99 (3 figs.). 
t This device is not new. Stops were fitted to objectives for this purpose by 
"VY enliam, circa 1850, examples of which are in the Cabinet of the Society. — Ed. 
