USE OF THE COM POUND MICROSCOPE 29 



When this is the case, precisely that circle of the object 

 which is seen is illuminated; and there is no useless marginal 

 light entering the microscope to cause glare. 



The iris diaphragm of the (focused) condenser determines 

 the used aperture of the condenser, which is one of the 

 most important adjustments to be made in microscopy. 



Fig. 8.- — -Diagram showing the different fields in the microscope, depending 

 on the diaphragm in the eyepiece. The lamp L illuminates a ground-glass 

 screen, close to which a diaphragm (source-diaphragm) bounds a circle, in this 

 case equal to the source-field Si. After leaving the color screen and the reflecting 

 prism, the rays from the source-field are focused by the condenser to form a 

 small circle S-2 on the object, in this case equal to the object-field. The rays 

 from the object-field, altered by having passed through the object, are focused 

 by the objective in the plane of the diaphragm of the eyepiece (magnifier pattern) 

 which they just fill, 1S3. They are then focused by eyepiece plus eye to form a 

 circle on the retina (not shown). (O and C represent aperture circles, not 

 fields.) 



This aperture must be filled with an even light. The 

 light-filled image of this aperture can be seen on the back 

 of the (focused) objective as the aperture circle of the 

 condenser; or briefly, condenser circle (Fig. 9). On the 

 back of the objective there can also be seen an outer 

 circle, the aperture circle of the objective (briefly, objective 

 circle)^ w^hich is usually determined, not by a special 

 diaphragm, but by the margin of the objective lenses. 



