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V . — A New Illuminating Apparatus for the Microscope. 
By Professor E. Abbe, of Jena. 
Two non-achromatic lenses, A a of the annexed figure, constitute 
the illuminating part of the arrangement or “ condenser,” to retain 
the term in common use. This portion of the apparatus pre- 
sents the form of a stout object-glass, and has a thick plano-convex 
lens uppermost, of more than hemispherical form. This part of 
the arrangement is set in a brass disk b b, by means of which it fits 
into the stage of the microscope from above. When once inserted 
into its place for use, it remains permanently fixed in the axis of 
the instrument. The plane upper surface a of the top lens lies in 
that case only a trifle lower than the plane of the brass disk b b, 
and as the latter, in its turn, is situated accurately in the plane of 
the stage, the general level remains unbroken, except by the 
narrow flat groove surrounding the surface of the lens. The focal 
length of the entire combination amounts to about 15 millimeters, 
and this cannot be materially increased — although it would in itself 
be of advantage — without involving dimensions of an inconvenient 
size. The superior focus lies only a couple of millimeters above 
the plane surface of the front lens. When, therefore, a slide 
having an object upon it is laid on the stage of the microscope, the 
object of its own accord is brought to lie almost in the focal point, 
g 2 
