106 Dr Brewster on Vision tlirougit Coloured Glasses^ 
may be made to coincide with the greatest accuracy, and conse- 
quently the performance of the instrument greatly improved. 
In small microscopes, the power of illuminating the ob- 
ject is very limited, from its proximity to the instrument. The 
liglit which it would naturally receive, is obstructed by the head 
of the observer, and by the body of the microscope ; and when 
the object is perfectly opaque, it is almost impossible to throw up- 
on it the requisite degree of light. In large microscopes, on the 
contrary, where the object is o^ie or two^ or even three feet from 
the object-lens, we may project upon it any quantity of light 
that we please. 
The ordinary microscopes, both single and compound, 
are incapable of being applied to objects placed in a cavity, or 
in the interior of a transparent crystal ; but in large microscopes, 
the depth of the cavity, and the thickness of the crystal, bear 
no sensible pro}X)rtion to the distance of the olject from the mi- 
croscope ; and the cavity, or any object which it includes, may 
be seen to the greatest advantage. 
G/J/i, In viewing an object of perceptible thickness, such as a 
fly, through the compound microscope, it is impossible to see 
the near and the remote parts at the same time, so that a num- 
ber of successive adjustments are necessary, and even then, we 
are imperfectly acquainted with its general form and outline. 
In large microscopes, however, the thickness of the object bears 
a very slight proportion to the conjugate focal distances of the 
object-lens, so that the instrument may at once be adjusted to 
all the parts of it that are within the field of view. 
When the object to be examined is an optical structure, sucli 
as that exhibited by plates of amethyst, the ordinary microscope 
is entirely useless, as the figure to be observed is produced by 
the action of every point of the transparent plate. When the 
microscope is large, however, the figure is seen with as much 
distinctness as if it had been formed by a plate of no other di- 
mensions but length and breadth. 
Such are a few of the advantages which we may confidently 
expect from the use of large microscopes. We would recom- 
mend them strongly to the attention of naturalists, whose pur- 
suits lead them to investigate the more minute phenomena of 
vegetable and animal lifce The portion of nature which has 
