The article 'Microscope'.

  • Ross, Andrew, 1798-1859.
Date:
[between 1830 and 1839?]
Catalogue details

Licence: Public Domain Mark

Credit: The article 'Microscope'. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    THE ARTICLE ‘MICROSCOPE.’ l^From the Penny Cyclopcedia.l MICROSCOPE, the name of an instrument for enabling the eye to see distinctly objects which are placed at a very •abort distance from it, or to see magnified images of small objects, and therefore to see smaller objects than would otherwise be visible. The name is derived from the two ' Greek words, expressing this property, umpog, small, and rVKtnriot, to see- So little is known of the early history of the microscope, and so certain is it that the magnifying pow’er of lenses • must have been discovered as soon as lenses were made, ' that there is no reason for hazarding any doubtful specula- ' tions on the question of discovery. We shall proceed there- ■fore at once to describe the simplest forms of microscopes, !to explain their later and more important improvements, and ' finally to exhibit the instrument in its present perfect state. In doing this we shall assume that the reader is familiar with tlie information contained in the articles Light, Lexs, Achromatic, Aberration, and the other subdivi- 'Sionsof the science of Optics which are treated of in this work. The use of the term magnifying has led many into a mis- ' conception of the nature of the effect produced by convex lenses. It is not always understood that the so-called mag- nifying pow^er of a lens applied to the eye, as in a micro- •acope, 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 he fakes, makes a change in the optical arrangement 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 arc totally unaware of its exercise. In the case just mentioned the bill would be read with eyes in a very different stale 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 suspi- cion, he w'ould presently find that the power of natural adjust- ment has a limit; for when his eye has airived 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 see the object gradually and dis- tinctly increase in apparent size by the mere continuance of the operation of approaching. Yet 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 pe- riods 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, and 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 combination of lenses. This description of magnifying power does not apply to such instruments as the solar or gas microscope, by which we look not at the object itself, but at its shadow or picture on the wall; and the description will require some modifi- cation in treating of the compound microscope, where, as in the telescope, an image or picture is formed by one lens, that image or picture being viewed as an original object by another lens. It is nevertheless so important to obtain a clear notion of the real nature of the effect produced by a lens ap- plied to the eye, that we will adduce the instance of spectacles to render the point more familiar. If the person who lias A