MI'CROSCOPE (Gr. mares, small, and skopeo, I see) is an instrument for enabling us to examine objects which are so srnall as to be almost or quite 'indiscernible by the unaided eye. Its early history is obscure; but as it is quite evident the property of magnifying possessed by the lens must have been noticed as soon as it was made, we are quite safe in attributing its existence in its simplest form to a period considerably anterior to the time of Christ. It is generally believed that the first compound microscope was made by Zacharias Jansen, a Dutchman, in the year 1590, and was exhibited to James I. hi Lon don by his astronomer, Cornelius Drebbel, in 1619. It was then a very imperfect instru ment, coloring and distorting all objects. For many years it was more a toy than a useful instrument, and it was not until the invention of the achromatic lens by Hall and Dollond, and its application to the microscope by Lister and others, that it reached the advanced position it now occupies atnong scientific instrunients.
An object to be magnified requires simply that it be brought nearer to the eye than when first examined, but as the focal distance of the eye ranges from 6 in. to 14 in.-10 in. being the average focal distanee—it follows that a limit to the magnifying power of the eye attained whenever the object to be examined is brought so near. If, however, we blacken a card, and pierce a hole in it with a tine needle, and then examine a minute object, as, for instance the wing of an insect held about an inch from the card, we shall see it distinctly, and that too magnified about ten thnes its size. This iS explained by the fact that the pin-hole limit§ the divergence of the pencil of rays, so that the eye cau converge it sufficiently on the retina to produce a distinct impression, which is faint; and did not the blackened card exclude all other light, it would be lost. If we now remove the blackened card without either removing onr eye or the object under examination, it will be found that the insect's wing is almost invisible, the unassisted eye being unable to see clearly an object so near as one inch; thus demonstrating the blackened card with the needle-hole in it to be as decided a magnifying instrument as any set of lenses.
By the apparent size of an object is understood the angle formed by two lines drawn from the center of the eye to the extremities of the object, which is larger when the object is nearer the eye than when further removed. This angle is called the antde of vision, and is quite distinct from the angle of the pencil of light, by which the object is seen. The focal length of a lens determines its magnifying power. The object to be examined is placed in its focus, so that the light which diverges from each point may, after refraction by the lens, proceed to the eye in lines as nearly parallel as is necessary for distinct vision. Thus, in fig. 1, AB is a double convex lens, in the focus of which we have drawn an arrow, EF, to represent the object under inspection.
The cones drawn from its extremities are portions of the rays of light diverg ing from these points, and falling on the lens. These rays, if not interrupted in their course by the lens AB, would be too divergent to permit their being brought to a focus upon the retina by the lenses which constitute the eye.
But as'they are first passed through the lens AB, they are bent into nearly parallel lines, or into lines diverging from some points within the limits of distinct vision, as from CD. Thus bent, these rays are received by the eye as if proceeding from the larger arrow CD, which we may suppose to be 10 in. from the eye, and then the ratio of the length of the virtual image to that of the real arrow (nearly 10 to 1) gives the magnifying power of the lens in question. The ratio of CD to EF is the same as that of IIG to KG. Now, EG is the distance of distinct vision, and KG the focal length of the lens, so that the magnifying power of a lens is obtained by dividing the distance of distinct vision (10 in. for most individuals) by its focal length. Thus, if the focal length of a leus be in., . 10 the mag-nifying power is = 40. This supposes that the distance between the eye and the lens is so small as not materially to interfere with the correctness of this statement.