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 may be so disposed that the two minerals have a definite relation between their crystallographic axes (parallel growth). The quartz typically occurs as angular patches; at other times it forms club-shaped, curved or vermiform threads (vermicular micropegmatite, myrmekite), and then some authors consider that the felspar has been corroded and the quartz fills up the spaces thus produced (quartz de corrosion of French petrographers). Micropegmatite is often so fine grained that even in the thinnest sections and with high powers it cannot be resolved into its components. This fine micropegmatite resembles threads, having a divergent arrangement. In some rocks the whole ground mass consists of such spherulitic growths of fibrous micropegmatite (see ); in their centres there is often a quartz or felspar crystal; the outer boundaries of the spherulites are not usually circular but irregular owing to the interlocking of adjacent spherulites at their margins (“granophyric structure”). Micrographic structures may occur in other minerals, e.g. quartz and garnet, cordierite, epidote or hornblende, augite and felspar, but are less common, and the name micropegmatite is usually reserved for aggregates of quartz and felspar.

 MICROSCOPE (Gr. , small, , to view), an optical instrument for examining small objects or details of such objects; it acts by making the angles of vision under which the images appear greater than when the objects themselves are viewed by the naked eye.

Microscopes are distinguished as simple and compound. A simple microscope consists of a single positive lens, or of a lens combination acting as a single lens, placed between the eye and the object so that it presents a virtual and enlarged image. The compound microscope generally consists of two positive lens systems, so arranged that the system nearer the object (termed the objective) projects a real enlarged image, which occupies the same place relatively to the second system (the eyepiece or ocular) as does the real object in the simple microscope. An image is therefore projected by the ocular from the real magnified image produced by the objective with increased magnification.

History of the Simple Microscope.—Any solid or liquid transparent medium of lenticular form, having either one convex and one flat surface or two convex surfaces whose axes are coincident, may serve as a “magnifier,” the essential condition being that it shall refract the rays which pass through it so as to cause widely diverging rays to become either parallel or but slightly divergent. Thus if a minute object be placed on a slip of glass, and a single drop of water be placed upon it, the drop will act as a magnifier in virtue of the convexity of its upper surface; so that when the eye is brought sufficiently near it (the glass being held horizontally) the object will be seen magnified. Again if a small hole be made in a thin plate of metal, and a minute drop of water be inserted in it, this drop, having two convex surfaces, will serve as a still more powerful magnifier. There is reason to believe that the magnifying power of transparent media with convex surfaces was very early known. A convex lens of rock-crystal was found by Layard among the ruins of the palace of Nimrud; Seneca describes hollow spheres of glass filled with water as being commonly used as magnifiers.

The perfect gem-cutting of the ancients could not have been attained without the use of magnifiers; and doubtless the artificers who executed these wonderful works also made them. Convex glass lenses were first generally used to assist ordinary vision as “spectacles”; and not only were spectacle-makers the first to produce glass magnifiers (or simple microscopes), but by them also the telescope and the compound microscope were first invented. During the Thirty Years’ War the simple microscope was widely known. Descartes (Dioptrique, 1637) describes microscopes wherein a concave mirror, with its concavity towards the object, is used, in conjunction with a lens, for illuminating the object, which is mounted on a point fixing it at the focus of the mirror. Antony van Leeuwenhoek appears to be the first to succeed in grinding and polishing lenses of such short focus and perfect figure as to render the simple microscope a better instrument for most purposes than any compound microscope then constructed. At that time the “compass” microscope was in use. One leg of a compass carried the object, and the other the lens, the distance between the two being regulated by a screw. Stands were also in use, permitting the manipulation of the object by hand. Robert Hooke shaped the minutest of the lenses with which he made many of the discoveries recorded in his Micrographia from small glass globules made by fusing the ends of threads of spun glass; and the same method was employed by the Italian Father Di Torre. Early opticians and microscopists gave their chief attention to the improvement of the simple microscope, the principle of which we now explain.

Position and Size of the Image.—A person with normal vision can see objects distinctly at a distance varying from ten, inches to a very great distance. Objects at different distances, however, are not seen distinctly simultaneously, but in succession. This is effected by the power of accommodation of the eye, which can so alter the focal length of its crystalline lens that images of objects at different distances can be produced rapidly and distinctly one after another upon the retina.

The angle under which the object appears depends upon the distance and size of the object, or, in other words, the size of the image on the retina is determined by the distance and the dimensions of the object. The ratio between the real size of the object y (fig. 1)



and the distance l, which is equal to the tangent of the visual angle w, is termed the “apparent size” of the object. From the figure, which represents vision with a motionless eye, it is seen that the apparent size increases as the object under observation is approached. The greater the visual angle, the more distinctly are the details of the object perceived. On the other hand, as the observer recedes from the object, the apparent size, and also the image on the retina diminishes; details become more and more confused, and gradually, after a while, disappear altogether, and ultimately the external configuration of the object as a whole is no longer recognizable. This case arises when the visual angle, under which the object appears, is approximately a minute of arc; it is due to the physiological construction of the retina, for the ends of nerve fibres, which receive the impression of light, have themselves a definite size. The lower limit of the resolving power of the eye is reached when the distance is approximately 3438 times the size of the object. If the object be represented by two separate points, these points would appear distinct to the normal eye only so long as the distance between them is at the most only 3438 times smaller than their distance from the eye. When the latter distance is increased still further, the two appear as one. Therefore when it is desired to distinctly recognize exceedingly small objects or details of such, they are brought as near as possible to the eye. The eye is strained in bringing its focal length to the smallest possible amount, and when this strain is long continued it may cause pain. When the shortest distance obtained by the highest strain of accommodation is insufficient to recognize small objects, distinct vision is possible at even a shorter distance by placing a very small diaphragm