Page:Optics.djvu/145

 The limiting value of $AQ$ is easily found to be $cF⁄c+F$, which gives for the extreme magnifying power

The greatest angle under which the image of a line of the length $c+F⁄F$ can be viewed is

It appears from the two last observations, that a long-sighted person derives most advantage from a simple microscope, but that a short sight enables one to view a minute object more closely, and to use a greater magnifying power with a given microscope.

It need hardly be said, that the shorter the focal length of a lens, or the greater its power, (see p. 68.) the more it will magnify.

When a very great power is required, it is not uncommon to use a minute spherule of glass, of water, or of colourless varnish, stuck in a needle-hole in a plate of metal, which should be ground hollow on both sides, so as to be as thin as possible, where the aperture is made. The distance of the principal focus of a sphere from its surface being only half the radius, the magnifying power of such an apparatus is very great.

The following Table, abridged in part from the Encyclopædia Britannica, gives the magnifying power of small convex lenses or