118 PHOTOMICROGRAPHY 



(b) Second method. With the condenser, low-power 

 objective, and eyepiece on the microscope. Rack the 

 condenser and objective up or down until an image of the 

 light can be seen ; move the mirror until the image lies 

 centrally across the field of view. . 



3. To centre the Condenser. Close the iris diaphragm 

 below the condenser to a pin-point. Fit a low-power 

 objective, f" or 1J", and low eyepiece, and endeavour 

 to focus the edge of the aperture by racking either the 

 microscope tube or condenser up or down. An Abbe 

 condenser does not give a sharp image of the diaphragm, 

 and when the iris is close beneath it, as in the standard 

 Continental substage, it is practically impossible to 

 focus the image even with 2" or 3" objectives. An achro- 

 matic condenser gives a sharp image, and, with the iris 

 further away from it, as is usual in the English substage, 

 focus can be obtained with a f " or higher objective. With 

 the margin of the iris as nearly as possible in focus, centre 

 the aperture by gently turning the adjusting screws on the 

 condenser ring, or. if they are fitted to a nose-piece instead, 

 centre the objective. 



If there is no means of bringing the optical axes of the 

 objective and condenser into alignment, and the latter 

 appears to be out of centre, turn it round in its mount, 

 and possibly a position will be found in which the centra- 

 tion is fairly accurate. 



When it is impossible to focus the iris below an Abbe 

 condenser with the objectives at hand, focus on to the 

 upper surface of the condenser with a 1" or f " objective, 

 until a dimly -defined dark spot appears in the field, and 

 about ^ of its diameter. Centre this as above. 



4. To Focus the Substage Condenser. Use the plane 

 mirror to reflect the light on to the condenser. Place 

 a slide on the stage and focus with a f or other low 

 objective and low-power ocular. Then rack the condenser 

 until an image of the radiant is visible across the 



