121
The limiting value of AQ is easily found to be cFc+F, which gives for the extreme magnifying power
c+FF, or cF+1.[1]
The greatest angle under which the image of a line of the length α can be viewed is
c+FcFα, or α{1F+1c}.
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.[2]
The following Table, abridged in part from the Encyclopædia Britannica, gives the magnifying power of small convex lenses or
- ↑ When the object is at the focus, and the image infinitely distant, the magnifying power is cF, one less than this.
- ↑ The smallest, and therefore the most powerfully magnifying spherules ever made, were some that Di Torre, of Naples, sent to the Royal Society. One of them was only 1144th of an inch in diameter, and was said to magnify 2560 times; but Mr. Baker, to whom they were given for examination, could not make any use of them, though he very nearly destroyed his eye-sight in the attempt.
