AMERICAN ASSOCIATION ADVANCEMENT OF SCIENCE. 635 
his really excellent achromatic condenser. The effici iency of his -— 
ratus will be vastly increased by adding the graduating diaphragm mad 
dá Collins and others in London and occasionally offered for sale in this 
n 
vate London dealers. At first sight it cmo uld seem that this apparatus 
ould not be used on stands of the *: Jackson ". model; but, by a little 
Far cci filing, it can be used on large ie of this style, as I have 
been accustomed to do for years. After using a graduating diaphragm in 
the ordinary microscopical work of natural history, the orthodox wheel 
à i 
changes of light, seems simply absurd. For use irae ERS - or 
arg 
phragm should be used on all stands to whieh it can be applied; dim 
i ati 
iris diaphragm as made by Beck. There is often some difficulty in getting 
the graduating diaphragm sufficiently near to the lenses in the small lens 
condensers, but none in the eye-piece condensers. 
The easiest and most fascinating use of the stereoscopie microscope is 
doubtless with opaque or translucent objects with the paraboloid or other 
means of Misi aaa illumination. In lighting transparent objects 
under the binocular we have only one new condition introduced, the ne- 
cessity of a wide horizontal illumination in order to give an even light 
over the whole of both fields. Focussing the condenser upon the object 
and gradually opening the diaphragm, we shall probably find, with a 1- 
inch of 259, the best definition and resolution accomplis ished just at the 
cones of light each having an angular width about one-half or one-third 
of that of a objective, and converging hori detta upon the object at 
an angle nearly as atas that of the chet, v hall have both fields 
fairly « m piede lighted, and no glare he same zu is attained by a 
stop with a Pei slit, giving a dr umi and narrow vertical 
pencil of lig 
This Bs may be applied with some dps even to instru- 
ments without €— by placing a disc like Fig. 1 of Plate 5, hav- 
ng an opening of suitable width, over the pele) to shape the cone 
of light from the concave abi ee r the regular wheel of apertures may 
be replaced by a somewhat larger one containing one or "-" openings of 
this shape. 
Next comes the spotted lens, which may be applied to any microscope 
and which will greatly increase its working power at an almost nominal 
