THE CONDENSER. 
69 
The concave mirror serves as a very low angled con¬ 
denser when the object is in its principal focus. Io 
give facility for this disposition, the mirror of the micro¬ 
scope must slide up and down the rod or “ tail ’ to 
which it is attached; and it is pointed out in Ballinger’s 
“ Carpenter” that with diffused daylight even the plane 
mirror can be used to “ condense.” 
A bull’s-eye may also be used as a condenser up to 
an angle of about 25 degrees ; to effect this we turn the 
convex slide of the lens towards the light, and bring the 
lens near to the object ; in such a case, looking at the 
back of the object and moving the bull’s eye to and 
from it, we can see when the latter is in the proper focus. 
Sometimes in the very lowest power work even the 
mirror or the bull’s-eye gives too small a held of light; 
m such a case we interpose at a short distance behind 
the object a piece of finely-ground glass, using parallel 
or slightly diverging rays from a paralleliser or bull’s- 
eye. This will be found a good help out of a difficulty 
which not seldom turns up in general work. 
The next form of condenser requiring attention is the 
ordinary achromatic made by all opticians. One of these 
may be used giving an angle of about 160 degrees, and 
it should be so constructed that the top element can be 
removed, the remainder giving angles suitable for 
objectives such as one-inch, two-thirds, or ordinary 
half-inch. 
The most useful of all types of dry condenser is 
represented by the achromatic of Zeiss, having numeri¬ 
cal aperture available to about -95. The front of this 
