object from edge to edge, and express the result as
_micra_ by reference to the recorded values for that particular optical
combination.

_Microscope Illuminant._--In tropical and subtropical regions diffuse
daylight is the best illuminant. In temperate climes however daylight of
the desirable quantity is not always available, and recourse must be
had to oil lamps, gas lamps--preferably those with incandescent
mantles--and electricity; and of these the last is undoubtedly the best.
A handy lamp holder which can be manufactured in the laboratory is shown
in Fig. 60. It consists of a base board weighted with lead to which is
attached the ordinary domestic lamp holder, and behind this is fastened
a curved sheet-iron reflector. An obscured metal filament lamp of about
16 candle power gives the most suitable light, and if monochromatic
light is needed, the blue grease pencil is streaked over the side of the
lamp nearest the microscope; the current is switched on and when the
glass bulb is warm, rubbing with a wad of cotton-wool will readily
distribute the blue greasy material in an even film over the ground
glass.

[Illustration: FIG. 60.--Electric microscope lamp.]

FOOTNOTES:

[1] Its importance will be realised, however, when it is stated in the
words of the late Professor Abbe: "The numerical aperture of a lens
determines all its essential qualities; the brightness of the image
increases with a given magnification and other things being equal, as
the square of the aperture; the resolving and defining powers are
directly related to it, the focal depth of differentiation of depths
varies inversely as the aperture, and so forth."

[2] Made by Mr. Otto Baumbach, 10, Lime Grove, Manchester.

V. MICROSCOPICAL EXAMINATION OF BACTERIA AND OTHER MICRO-FUNGI.

APPARATUS AND REAGENTS USED IN ORDINARY MICROSCOPICAL EXAMINATION.

The following comprises the essential apparatus and reagents for routine
work with which each student should be provided.

1. India-rubber "change-mat" upon which cover-glasses may be rested
during the process of staining.

2. Squares of blotting paper about 10 cm., for drying cover-slips and
slides.

(The filter paper known as "German lined"--a highly absorbent, closely
woven paper, having an even surface and no loose "fluff" to adhere to
the specimens--is the most useful for this purpose.)

[Illustration: FIG. 61.--Disinfectant Jar.]

3. Glass jar filled with 2 per cent. lysol solu