Beck, on a new kind of Illumination, 
37 
By means of a slot {e, fig. 1) in the side of the adapter the 
thin glass may be readily removed for the purpose of being 
wiped, as its perfect freedom from dust or smear is most 
essential. 
I don^t know of any illumination connected with the 
microscope that requires more care and thought in its use 
than this. We have to consider, in the first place, that all 
the light thrown upon the object passes through the object- 
glass, which consequently regulates the direction of the illu- 
mination, and thus the obliquity can never exceed the angle 
of the pencil of light admitted by the aperture of the same 
riG. 1. Fig. 2, 
object-glass ; and, secondly, it must be remembered that an 
object, or any part of any object, that lies in a plane at right 
angles to the axis of the body of the microscope^ and possesses 
a reflecting power, will merely return the light into the 
instrument, not only giving a more or less milky appearance 
to the picture, but also a very deceptive representation of the 
specimen. 
A striking illustration of this fact is given when a piece of 
thin glass is over the object, this so thoroughly reflects the 
light that little can be seen beyond it, and especially so with 
object-glasses of large aperture. With this illumination 
therefore all objects should be uncovered, and even then 
success is not always certain. I have tried to get a view of 
the tracheal vessels in the flea by this method, but the horny 
plates reflect so much light that hardly anything can be seen 
beneath them. The proboscis of the blow-fly, when simply 
