Microscope And Micrometer. The 6ize of objects is expressed usually in parte of an inch, as one-fifteenth, one-hundredth, or one-thousandth part of an inch. In order to ascertain this actual size, various modes are employed. The most simple is to place on the. stage of the microscope a piece of glass which has fine lines ruled on it, perhaps an hundredth part of an inch apart, and comparing these divisions with a rule seen by the naked eye. In this way we may learn how much the microscope magnifies. If we compare the apparent size of an object on the stage with the rule as before, and divide that apparent size by the magnifying power of the instrument, we shall learn exactly how large it is. The piece of glass with the fine lines ruled on it is called a micrometer, which means an instrument to measure small things, as the word microscope means an instrument to observe small things. If we find by the micrometer that the microscope magnifies two hundred and fifty times, and a small object at which we are looking appears an inch in diameter, wo know that it really is but the two hundred and fiftieth part of an inch in diameter.