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An object one hundredth of an inch in diameter, or of which it would take one hundred placed side by side to make an inch, is about the smallest thing that can be easily seen by the unassisted eye. Take a piece of card and punch a little hole through it with the point of a small needle, hold it towards a lamp or a window, and you will see the light through it.
This hole will be about the size just mentioned, and you will find that you can see it best and most distinctly when you hold it at a certain distance from your eye; and this distance will not be far from ten inches, unless you are near-sighted. Now bring it towards your eye and you will find it becomes blurred and indistinct. You will see by this experiment that you cannot see things distinctly when held too close to your eye, or in other words, that you cannot bring your eye nearer to an object than eight or ten inches and see it well at the same time.
You could see things much smaller than one hundredth of an inch if you could get your eye close enough to them. How can that be done? By a microscope? yes, but what is that? This name comes from two Greek words that mean "to see small things;" and a microscope is an instrument by which your eye can get very close to what you want to see.