236 J. Lawrence Smith on Yhe Inverted Microscope. 
angles a, b, c, d, respectively 55°, 107$°, 524°, 145°, the angles 
being of such dimensions, that a ray of light passing into the 
prism in the directions shown by the arrows, and perpendicu- 
lar to the surface a d, after undergoing total reflection from the 
inner surfaces a band 6 ¢ (on both of which the light strikes at 
an angle much less than forty-five degrees) will pass out perpen- 
dicular to the surface ed. If the line c be followed, it will be 
readily seen how.a ray of light passing through the object-glass 
, descends into the prism, and passes out of it upwards through 
the eye-glass D, the tube of which is inclined to the perpendicu- 
lar 35°. The other parts of the instrument are understood by 
simply looking at the figure. E is a heavy support.that revolves 
en another support H, which carries a column I, on which are 
placed the mirror, diaphragm, &c. .The prism used has each 
side nearly an inch in length, and little less in width, which 1s 
about the most convenient size. The arrangement for adjusting 
the focal distances is somewhat peculiar, and is readily under- 
stood by reference 4. 
There are three tubes (the outer 
one of which is F) that slide on 
each other, the inner is fastened 
to the plate O, the second tube 
has a projecting collar, on the 
under surface of which rest the 
extremities of two springs y, and 
on the upper surface two points 
of the lever X, which is moved i) 
by means of the screw T. The plate O is fastened on to the top 
of prism by the binding screw L (fig. 1), that readily allows of 
the plate being detached at pleasure, which it is necessary 0 do 
at times in order to wipe the upper surface of the prism. The 
way in which the observer operates, is to screw one or other 0 
the object-glasses to a small cap K (fig. 4) that simply rest on t@ 
the upper end of the outer tube F', which is readily moved UP 
and down by the finger for the coarser adjustment, while the 
minute adjustment is obtained by moving the screw T.- 
This description suffices to make it clearly understood how the 
instrument is used, and the conveniences arising therefrom. — 
examining an object with this microscope, the object is arran 
in the ordinary way; when liquid it is placed in a watch glass oF 
such glass cells as are convenient touse. Inem loying re-agents 
they can be added, and their effects watched immediately, for it 
is readily seen how the eye guides the manipulations on the stag® 
and looks into the instrument almost at one and the same time; 
& circumstance that facilitates and renders highly satisfactory 
rience has cone 
all such operations, as nearly two years ex 
pane. Sp ¥ 
ee a Aoi angi a en 
