ILLUMINATING APPAEATUS 29 



object ; the mirror will require to be adjusted in order to direct 

 the light through the condenser. If the lamp be turned round 

 so that the edge of the flame is opposite the mirror, it makes 

 an easier object to focus, and its image will appear as a slit 

 across the field of view. When focussed the lamp may be turned 

 round, with the flat side of the flame facing the mirror, thus 

 illuminating the whole field. It is found in practice that the best 

 resolution is obtained when the source of light is almost, but not 

 quite, in focus. 



The accuracy of centring of the simple Abbe condenser is not To centre 

 important. The image that it gives is not accurate, and it is col'denS 

 generally sufiicient to move the mirror slightly till the image of 

 the object does not move from side to side, while the body 

 of the microscope is being focussed up and down in a similar 

 manner to that described on page 22 for setting the mirror 

 when used alone. Microscopes fitted with this condenser are 

 frequently not provided with a centring adjustment to the 

 substage. 



When using the achromatic or immersion condenser centring 

 is a matter of importance. The iris diaphragm is placed in a 

 position at such a distance below the condenser lenses that if 

 the condenser be moved downwards the source of light will be 

 put out of focus and an image of the small aperture in the iris 

 diaphragm can be sharply focussed. When doing this the iris 

 diaphragm should be closed to its smallest aperture, so that 

 the image is of a sufficiently small size to be seen in the field of 

 view. The condenser may then be moved by the centring 

 screws until the image of the diaphragm is in the centre of the 

 field of view. If the image of the small aperture is so far out of 

 centre that it is not in the field, the aperture can be enlarged 

 until its edge begins to appear. When the condenser has been 

 thus centred it should be moved upwards till the image of the 

 lamp is again in focus on the object, and the mirror readjusted 

 if the light is not in the centre of the field. 



The achromatic condenser is now in the best position for use 

 with a 1/3-inch (8-mm.), 1/6-inch (4-mm.), or 1/12-inch (2-mm.) 

 object glass, though slightly better resolution is obtained if the 

 source of light is a little out of focus. If it is required to use a 

 2/3-inch (16-mm.) object glass, the condenser can be focussed 

 down to give an evenly illuminated field, being brought back 

 into focus when a higher power is used. 



The substage condenser having been focussed and centred. Effect of 

 the eyepiece of the microscope should be removed, and the cEi^^iris*^ 

 effect of opening and closing the iris diaphragm be observed by diaphragm of 

 looking down the tube of the microscope. This, effect is best 

 observed when an object glass with a fairly large aperture such 

 as 1/6-inch (4-mm.) is used. When the iris diaphragm is fully open 

 the back lens of the object glass will be completely filled with 



