THE MICROSCOPE. 159 
coming through the specimen is lost, as none of its rays are deflected 
before reaching the lens, 
What is an Abbé condenser ? 
> It is a compound illuminating lens mounte. underneath the stage of 
the microscope. It gives a cone of rays that is very broad and short, 
and that issue from a very wide angle of aperture. In this way shadows 
that are caused by diffraction are made to disappear in the illuminated 
picture. 
What points are necessary in a good microscope ? 
It must magnify sufficiently. 
It must give a correct and well-defined image. 
Lastly, it must analyze an object into its simplest component parts. 
Upon what does the magnifying power of a lens depend ? 
Upon its focal distance or distance from the object. The shorter the 
focal distance the greater the magnifying power. 
Upon what does the sharpness of the picture depend ? 
Upon the exact spherical and chromatic correction of the objec. 
glass. 
What is the most important factor in determining the defining 
power of a lens ? 
The angle of aperture. 
Whatis the angle of aperture ? 
Imagine a line drawn through the front plane of the lens so that it 
connects the most distant points of its circumference. This is the 
diameter. Also imagine that the object to be observed is a single 
point. Connecting the ends of the lens’ diameter with this object 
point, the angle formed at the object point is the angle of aperture. 
The wider the angle of aperture the more rays of light can enter 
the lens, and the more light the better the definition. 
Therefore, a lens of a wide angle of aperture gives better resolving 
powers than one of a narrow angle. 
