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Un microscopio simple es aquel que solo utiliza un lente de aumento. El ejemplo más clasico es la lupa. El microscopio óptico estándar utiliza dos sistemas de lentes alineados.
El objeto por observar se coloca entre el foco y la superficie de la lente, lo que determina la formación de una imagen virtual, derecha y mayor cuanto mayor sea el poder dióptrico del lente y cuanto más alejado esté el punto próximo de la visión nítida del sujeto.
El holandés Anton Van Leeuwenhoek construyó microscopios muy eficaces basados en una sola lente. Esos microscopios no padecían las aberraciones que limitaban tanto la eficacia de los primeros microscopios compuestos, como los empleados por Robert Hooke, y producían una ampliación de hasta 300 veces; gracias a ellos Leeuwenhoek fue capaz incluso de describir por primera vez las bacterias.
The simple microscope
This simplest of all microscopes is nothing more than a convex lens, whose focal distance is extremely short. The rays coming from the object will, after their refraction, fall parallel upon the eye, and, consequently, make distinct vision. Therefore, a minute object, seen distinctly through a small glass lens by the eye put close to it, appears so much greater than it would to the naked eye placed at a greater distance. To illustrate this, let us suppose the focal distance of the glass to be half an inch, and the distance eight inches, the usual distance at which we view minute objects, then the object may be said to be magnified as much as eight inches exceed the small space, or the focal distance of the lens that is, in the proportion of sixteen to one, or sixteen times. If the focal distance of the lens were one fifth of an inch, the magnifying power would be forty times if one tenth of an inch, eighty times and if it were one twentieth of an inch, the diameter of any object would be magnified one hundred and sixty times, which is found by dividing eight inches by the focal length of the lens, 8 ÷ 1/20 = 160. The surface of the object will, of course, be found by multiplying the diameter into itself, which produces 25,600 times and the solidity or bulk would be magnified, 4,096,00 times, that is, the surface multiplied by the diameter.
The performance of the single microscope depends, in a great measure, on the clearness and purity of the glass of which it is made, and on the accuracy with which it is polished, so as to keep it of a true spherical figure. When completed that is, when ground and polished it should be as thin as it can possibly be rendered with a sufficient aperture. When the lens is thick, approaching to the figure of a globe, it is not as transparent as when thin, and the field of view at the edges is partly distorted. And it must be of a sufficient diameter of aperture, that the eye may take in a moderate field of view, and that there may be as little deficiency of light as possible.
Lenses have been made whose focal length did not exceed 1/40, 1/50, or 1/60 of an inch but such high powers are difficult to be used. Sir D. Brewster has remarked that “we cannot expect any essential improvement in the single microscope, unless from the discovery of some transparent substance, which, like the diamond, combines a high refractive power with a low power of dispersion.” In correspondence with this suggestion, the diamond has been of late years formed into lenses by Mr. Pritchard, of London. The first diamond lens was completed at the end of 1824, and he succeeded in finishing the first diamond microscope in 1826.
The focal distance of this magnifier, which was doubling convex, is about 1/30 of an inch. The principal advantages of employing diamonds in the formation of microscopes arise from the naturally high refracting power they posses, by which we can obtain lenses of any degree of magnifying power, with comparatively shallow curves. The indistinctness occasioned by the figure of the lens is thus greatly diminished, and the dispersion of color in the substance being as low as that of water, renders the lens nearly achromatic. Mr. Pritchard has formed lenses of sapphire and other precious stones, but they are not preferable as compared to the diamond.
In mounting the diamond and sapphire lenses there are advantages which glass lenses do not possess. Their extreme hardness enables them to be burnished with brass settings, which is very difficult with those of glass. This facility of mounting renders them more extensively useful in experimental researches, from their capability of being applied in every possible way with regard to the object, the light, or the eye. But it is evident that such lenses, both from the difficulty of grinding and polishing, and from the costliness of material, must be very expensive.
There are various simple methods of procuring small lenses for microscopes. Take a small slip of window glass, about one tenth of an inch broad melt it in the flame of a lamp, then draw it out into fine threads, then hold one of these threads with its extremity in or near a flame, till it runs into a globule. The globule may then be cut off and placed above a small aperture, so that none of the rays which it transmits pass through the part where it is joined to the thread of glass. Lenses are frequently useful where better microscopes are not at hand. Take up a drop of water on the point of a pin, and place it in a small hole in a thin piece of brass, about 1/20 of an inch in diameter. The hole should be in the middle of a small spherical cavity, about one seventh of an inch in diameter, and a little more than half of the thickness of the brass, which should not exceed 1/16 of an inch in thickness.
On the opposite side of the brass should be another spherical cavity, half as broad as the former, and so deep as to reduce the circumference of the small hole to a sharp edge. The water being placed in these cavities will form a double convex lens with unequal convexities, which will produce a pretty high magnifying power. A better substitute for water is a drop of very pure and viscid turpentine varnish, which may be taken up on the point of a piece of wood and dropped upon a piece of thin and well polished glass. Sir David Brewster describes the following as the best method of constructing fluid microscope
Take Canada balsam, castor oil, or pure turpentine varnish, and drop either of them on a piece of glass, the surface of which are parallel, when planoconvex lens will be formed. Their power may be varied by the quantity of the fluid employed, or by allowing the plate of glass to be horizontal with the drop above or beneath it if the plate be uppermost, the gravity of the fluid will make it more complex if the drop be above the plate, the lens will be flattened. When the turpentine is used, it soon becomes indurate, and, if kept from dust, very durable. Sir David said in his letters that he has made both the object and eye lenses of compound microscopes in this manner, which performed extremely well, and lasted a considerable time.
A single reflecting microscope may be formed by a concave speculum, having the object placed on is axis, and nearer to the surface of the reflector than the focus, when an enlarged view of the object will be seen on looking into the mirror. This instrument may be employed to enable a person to view his own eye, and will show a magnified representation of the ball, the pupil, the iris, and the ramifications of the blood vessels. On the same principle, if the reflector be large, for example, six inches in diameter, the whole head and face may be seen magnified three or four times in length and breadth, and above ten times in surface. There is a species of lens, sometimes called the Codington lens, formed by a piece of glass nearly half an inch in thickness. The upper and lower surfaces are convex. The sides are hollowed out, giving the lens somewhat that shape of an hourglass, and reducing the stem to a very small size. These lenses are of a very short focal distance, have a great magnifying power, and serve either as single microscopes, or for the object glasses of compound ones.