THE MICROSCOPE. 
against which the observer must be on his guard. The effects of 
these are to surround the outline of the object with coloured fringes. 
By limiting the illumination as far as it is practicable to the object 
itself, so as to avoid the transmission of any light through the 
opening of the slide, except what may pass through the object,, 
this effect may be diminished or avoided. 
Indeed, for many reasons, it is advantageous to prevent any 
light from passing through the slide, or through the opening of 
the stage, except what is employed in illuminating the object. 
All such light is liable to fall in greater or less quantity upon the 
object-glass, and, passing through it, has a tendency to render the 
image obscure and confused. For this reason, all extraneous 
light whatever should be as far as possible excluded from the 
space around the microscope, for all objects on which such light 
falls will reflect a part of it, some of which may fall upon the 
object-glass. 
50. "When the light of the sky or clouds is used, an aperture 
may be made in a window-shut ter for its admission, all the other 
windows of the room being closed, and the light proceeding 
from the aperture being received upon the mirror or lens, by 
which it is directed and condensed upon the object. The light 
of a white cloud, strongly illuminated by the sun, is generally 
considered the best form of day-light which can be used, and 
that of a blue serene sky the worst. Observers differ as to the 
direct light of the sun, some maintaining that in no case what¬ 
ever should it be used, while others give it a preference for 
minute objects seen under high powers, and therefore requiring 
intense illumination. 
The light reflected from a white wall upon which the sun 
shines is a good source of illumination. 
51. If artificial light be used with low powers, a common sperm 
candle will serve well enough, but means should be adopted to- 
prevent the flickering of the flame. 
An argand lamp, however, is, in all cases, preferable, as giving 
a steady invariable light. It will be improved if good olive oil 
be used instead of the fish oil. 
The flame produced by the liquid known as camphine is 
especially pure and white, and well fitted for microscopic- 
researches. 
"Whatever be the artificial light used, it ought to be surrounded 
with a shade, and so placed as to fall only upon the mirror or 
lens by which it is directed to and condensed upon the object. 
52. It is advantageous to protect the eyes of the observer from 
extraneous light: the most simple and convenient method of 
effecting which is by a circular blackened pasteboard screen 
46 ' 
