IIICHOSCOPE, the name of an instrument for enabling the eye to Dee distinctly objects which are placed at a very short distance from It, or to Nee magnified images of small objects, and therefore to see smaller object. than would otherwise be visible. The name is derived from the two Greek words, expressing this property, isorpSo, small, and 'serifs, to K.
So little is known of the early history of the microscope, and so certain is it that the magnifying power of lenses must have been dis covered as soon as lenses were made, that there is no reason for hazard ing any doubtful speculations on the question of discovery. We shall proceed therefore at once to describe the simplest feline of micro scopes, to explain their later and more Important improvements, mil finally to exhibit the instrument in its present perfect state.
In doing this we shall assume that the reader is familiar with the information contained in the articles Limn, LENS, ACIIROXATIC, ABER RATION, and the other subdivisions of the science of Optics which are treated of in this work.
The use of the term magnifying has led many into a misconception of the nature of the effect produced by convex lenses. it is not always understood that the so-called magnifying power of a lens applied to the eye, as in a microscope, is derived from its enabling the eye to approach more nearly to its object than would otherwise be compatible with distinct vision. The common occurrence of walking across the street to read a bill is in fact magnifying the bill by approach ; and the observer, at every step ha takes, makes a change in the optical arrange ment of his eye, to adapt it to the lessening distance between himself and the object of his inquiry. This power of spontaneous adjustment is so unconsciously exerted, that unless the attention be called to it by circumstances, we are totally unaware of its exercise.
In the case just mentioned the bill would be read with eyes in a very different state of adjustment from that in which it was discovered on the opposite side of the street, but no conviction of this fact would be impressed upon the mind. If however the supposed individual should
perceive on some part of the paper a small speck, which he suspects to be a minute insect, and if he should attempt a very close approach of his eye for the purpose of verifying his suspicion, he would presently find that the power of natural adjustment has a limit; for when his eye has arrived within about ten inches, he will discover that a further approach produces only confusion. But if, as he continues to approach, he were to place before his eye a series of properly arranged convex lenses he would sea the object gradually and distinctly increase in apparent size by the mere continuance of the operation of approaching. I et the glasses applied to the eye during the approach from ten inches to one inch, would have done nothing more than had been previously done by the eye itself during the approach from fifty feet to one foot. In both cases the magnifying is effected really by the approach, the lenses merely rendering the latter periods of the approach compatible with distinct vision.
A very striking proof of this statement may be obtained by the following simple and instructive experiment. Take any minute object, a very small insect for instance, held on a pin or gummed to a slip of glass ; then present it to a strong light, and look at it through the finest needle-hole in a blackened card placed about an inch before it The insect will appear quite distinct, mid about ten times larger than its usual size. Then suddenly withdraw the card without disturbing the object, which will instantly become indistinct and nearly invisible. The reason is, that the naked eye cannot see at so small a distance as one inch. But the card with the hole having enabled the eye to approach within an inch, and to see distinctly at that distance, is thus proved to be as decidedly a magnifying instrument as any lens or com bination of lenses.