ABSTRACT
As regards the exact positioning of the condenser for illumination according to Kohler, l'\:cept for very low magnification objectives, remember that it is close to the specimen and it must be determined each time for each objective used. This operation is carried out as follows:
1) the specimen is brought into focus; 2) the diaphragm of the condenser is
opened up completely; 3) the field diaphragm of the micro-
scope is closed down completely. If its axis does not match the instrument's, it is centered by manipulating the centering thumb screws of the condenser;
4) the condenser is raised or lowered until the field diaphragm's image is seen clearly in the observation area of the micro-
scope; in other words, the inside edge of the iris field diaphragm must be in focus;
5) the field diaphragm is opened up only until it completely disappears from the observation area of the microscope. Opening it up more is not necessary; moreover, doing so would create a ray of light that is wider than the field being observed and that would generate diffused light on the specimen, thus decreasing image contrast;
6) the diaphragm of the condenser is closed down until correct image contrast is achieved. In fact, the diaphragm is used to increase (by closing it down) or decrease (by opening it up) image contrast and not to regulate the quantity of light. A good approximation of the correct aperture of the diaphragm can be found by multiplying the numerical aperture of the objective by 0.7.
