tube at the other end
_B'_ of the stage allows exit to the waste water. By raising the
temperature of hanging-drop preparations, etc., placed upon it, above
that of the surrounding atmosphere, the warm stage renders possible
exact observations on spore germination, hanging-drop cultivations, etc.

[Illustration: FIG. 52.--Warm stage.]

A better form is the electrical hot stage designed by Lorrain Smith;[2]
it requires the addition of a lamp resistance and sliding rheostat, also
a delicate ammeter reading to .01 of an ampere. It consists of a wooden
frame supporting a flat glass bulb with a long neck bent upward at an
obtuse angle (Fig. 53). The bulb is filled with liquid paraffin, which
rises in the open neck when expanded by heat. The neck also accommodates
the thermometer. Two coils of manganin wire run in the paraffin at
opposite sides of the bulb (outside the field of vision), coupled to
brass terminals on the wooden frame by platinum wire fused into the
glass. The resistance of the two coils in series is about 10 ohms. A
current of 2-1/2 amperes is needed, and is conducted to the coils in the
stage through the rheostat. With the help of the ammeter any desired
temperature can be obtained and maintained, up to about 200 deg. C. If
immersion oil contact is made between the top lens of the condenser and
the lower surface of the bulb, this stage works very well indeed with
the 1/12-inch oil immersion lens.

[Illustration: FIG. 53.--Lorrain Smith's warm stage.]

_Dark Ground or Paraboloid Condenser._--This is an immersion substage
condenser of high aperture by means of which unstained objects such as
bacteria can be shown as bright white particles upon a dense black
background. The central rays of light are blocked out by means of an
opaque stop while the peripheral rays are reflected from the
paraboloidal sides of the condenser and refracted by the object viewed.
To obtain the best results with this type of condenser a powerful
illuminant--such as a small arc lamp or an incandescent gas lamp--is
needed, together with picked slides of a certain thickness (specified
for the particular make of condenser but generally 1 mm.) and specially
thin cover-glasses (not more than 0.17 mm.) The objective must not have
a higher NA than 1.0, consequently immersion lenses must be fitted with
an internal stop to cut down the aperture.

_Micrometer._--Some form of micrometer for the purpose of measuring
bacteria and other objects