ON THE CHOICE OF A MICROSCOPE. 157 
coming beneath them, but so obliquely, that none of it enters 
the object-glass but that which is interrupted by the object. 
5th. Mr. Brookes’ double nose-piece is a most useful piece 
of mechanism attached to the end of the microscope body, by 
which any two object-glasses can be screwed on to it at once, 
and rapidly changed with each other. 
6th. The double arm to the plano-concave mirror is a great 
improvement on the old method of supporting it, since it 
allows it to be so extended as to cast a very oblique light on 
objects, as well as to be raised near them without any 
preparatory movement. 
7th. The separation of the outer and inner lenses of the 
lower powers was a happy idea first carried out by Mr. T. 
Ross, by which the greatest flatness of field and penetration 
are secured, and which has been adopted with signal success 
in his recently invented 38-inch distinct combination object- 
glass, which embraces in one field of view comparatively large- 
sized objects, such as flowers, ferns, flower-seeds, and mosses. 
8th. Mr. Wenham’s binocular arrangement, with double 
eye-pieces and prism, through which objects under low and 
medium powers are seen to stand out with solid stereoscopic 
effect, is the greatest recent invention of the modern micro- 
scope. This striking result is effected by means of a prism 
placed immediately over the object-glass, and which reflects 
one-half of the rays that proceed up the ordinary body of the 
microscope into another body attached at a certain inclination 
to it. The practical benefit of this new arrangement is, that 
it affords not a mere claptrap exhibition of the objects 
submitted to it, but gives real relief to the eyes by calling 
both into exercise, and allows the details of an object in their 
relation one to another to be far more clearly distinguished, 
than they could possibly be when the single body and eye- 
piece alone are employed. The prism above the object-glasses 
can be drawn aside whenever it is required to exclude the 
light from passing up the slanting body, and to use the per- 
pendicular one as an ordinary microscope, and which of 
course will often be the case in the examination of flat objects 
by low, and of test objects by the highest powers. 
Now I have-been particular, even at the risk of being 
tedious, in drawing attention to these various improvements 
which have been effected during the last few years in the 
achromatic microscope, because they are just those which 
make all the difference in the world in the pleasure of using 
it, and constitute it quite another instrument from what it 
was fifteen years ago. Fortunately, the binocular body, the 
dark-ground illuminator, and the 3-inch distinct combination 
