An instrument of which there are various construe-tions,'usually applied to telescopes and microscopes, for the purpose of measuring minute bodies, or small angles formed by bodies at a remote distance, by which their real magnitude is obtained. To the modern introduction of this instrument for the use of the astronomer, and to the improvement of the telescope, may be attributed our present accurate and extensive acquaintance with the universe of matter; while, from the perfection to which the microscope has been brought, an equal acquaintance with the organization of minute bodies may be expected. By the application of the micrometer to the latter instrument, the power of the naturalist is materially extended; while the micrometer is of the utmost value for trigonometrical surveys, and in military and naval operations.