EXAMPLES OF PROCEDURE. 
87 
the image of the object. If the radiant-image does not 
fall on the held the light is to be moved till it does ; but 
the condenser is not to be shifted by the centring 
screws. When the image of the radiant falls on the 
object, rack the condenser till, as above, the light-image 
is seen sharp across the image of the object also sharp. 
The object is now in the plane of the conjugate foci of 
objective and condenser, and the image is, so far as 
focus goes, “ critical.” The condenser is now central 
and in focus. 
Notes. —If an od lamp is used, the wick should be 
turned with its edge towards the condenser ; for the 
very lowest-power work the side of the wick may be 
presented. The image of a wick-flame will appear as a 
streak of light down the middle of the field, the incan¬ 
descent area of a lime will appear as a round, or oblong, 
area; the size of either will depend, coeteris paribus , on 
the distance of the radiant from the condenser, and on 
the focal length of the condenser. A high-angled con¬ 
denser, being presumably of short focal length, will give 
a small image of the radiant, and for low-power work, 
such as we propose to treat first, it is generally neces¬ 
sary to remove from the condenser its top element, 
usually hemispherical. In any case the condenser, as 
it is to be used, is to be focussed as described. If the 
image of the object, as seen through the objective, is 
“ misty,” or flooded with light, there is too much angle 
being used for the objective, and the condenser must be 
shut down till the image is freed from the mist. 
In every case the light and condenser must be 
