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6
Prior Scientific
7. Setting K
ö
hler Illumination
A.
Follow the procedure in section 6 to set up the instrument for your eyesight.
B.
With a specimen in sharp focus using the 10x objective, close the field diaphragm
until it impinges on the field of view, Figure 1, Picture A.
Figure 1
C.
Focus the condenser using the side mounted rack and pinion controls until the
leaves of the diaphragm are in sharp focus, Figure 1, Picture B.
D.
Using the condenser centration controls move the diaphragm into the centre of the
field of view, it may help to open it until it is nearly touching the outside of the
field to attain perfect centration, Figure 1, Pictures C & D.
E.
Fully open the field diaphragm.
F.
Fully open the condenser diaphragm and then slowly close it until you see the
contrast within the image increase. If you then remove an eyepiece and look
directly down the tube from a distance of 20-30cm you should see an image similar
to picture D in Figure 1. The aim here is to have the “bright” area occupying
approximately 70% of the total field. The amount it occupies will change
dependant on the objective lens in use. If you have set up the instrument using the
10x objective (recommended) then as you increase the magnification this
diaphragm will need opening to optimise the contrast and resolution. Often it is set
for the objective which is either most frequently used or most critical in terms of
resolution and left in that position