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once. They frequently shift suddenly from one quarter of the horizon to another, and then come again to the former point. A whirlwind is formed when gusts of wind come from different quarters at the same time, and meeting in a certain place, the air acquires a rotatory, or screw-like motion, round an axis; and this axis is sometimes stationary, and at other times moves on in a particular direction. A rotatory motion is given to dust, sand, water, part of a cloud, and sometimes even to bodies of great weight and bulk : carrying them either upwards or downwards, and lastly scattering them about in different directions. The waterspout is an extraordinary aqueous meteor, most frequently observed at sea. It commonly begins by, a cloud, which appears very small, and which mariners call the squall ; which augments in a little time into an enormous cloud of a cylindrical form, or that of a reversed cone, and produces a noise like an agitated sea,sometimes emitting thunder and lightning, and also large quantities of rain or hail, sufficient to inundate large vessels,.-overthrow trees and houses,--and every thing which opposes its violent impetuosity. Both this and the whirlwind are meteors arising from the same general cause, and are explicable upon principles, furnished by electrical experiments and discoveries.
CHAP. VII. -OPTICS. 1.
OPTICS is the science of vision; it treats of the whole doctrine of light and colours, and all the phenomena of visible objects. It is divided into three parts : (1.) Cutoptrics, which explains the properties of reflected light, and particularly that which is reflected from mirrors. (2.) Dioptrics, which considers the different refractions of light *passing through different media, as air, water, glass, &c. and especially lenses. (3.) Perspective, or the art of drawing the representation of any visible object on a plane surface, in such a manner as it would appear on some transparent surface, interposed between an object and the eye of an observer. Single particles of light, succeeding each other in a right line, constitute a ray of light, considered in a mathematical sense; but, physically speaking, a ray is the least part of light on which alone experiments can be made, A slender portion of rays separated from the rest, is called a pencil of rays. Pencils of rays are either conical or cylindrical; the axis of a pencil being the same with the axis of the cone or cylinder. By a medium, is meant any pellucid or transparent body, which suffers light to pass through it, as water, air, and glass.
2. Parallel rüys are those which move always at the same distance from each other. If rays continually recede' from each other, they are said to diverge; and if they continually approach towards each other, to converge. The point at which converging rays meet, is called the focus. The point towards which they tend, but which they are prevented from coming to, by some obstacle, is called the imaginary focus. When rays, after passing through one mediuin, on entering another medium of different density, are bent out of their former course, and made to change their direction, they are said to be refracted. When they strike against a surface, and are sent back again from the surface, they are said to be reflected. A lens is glass ground into such a form, as to collect or disperse the rays of light which pass through it. These are of different shapes, and from thence receive different names. A planoconvex has one side flat, and the other convex; a plano(concave is flat on one side, and concave on the other; a double convex, is convex on both sides ; a double concave, is concave on both sides; a meniscus, is convex on one side, and concave on the other, like the common watchglass. A line passing through the centre of a lens is called its axis. A prism, is a triangular piece of glass, which has the power of separating the rays of light into their component parts.
3. Bodies with polished surfaces, which reflect the rays of light copiously, are called mirrors. These are generally smooth plates of glass, tinned or quicksilvered on the back part, and called looking-glasses. Mirrors are either plane, convex, or concave. The first sort reflects the rays of light in a direction exactly similar to that in which they fall upon it, and therefore represents bodies of their natural magnitude. A ray coming from any luminous body, which falls upon a reflecting surface, is termed the
incident ray. A ray of light striking perpendicularly upon a plane inirror, is reflected back in the same direction; but those rays which strike it obliquely, are reflected back, in an opposite direction, but with the same obliquity; consequently the angle of reflection is exactly equal to the angle of incidence.'
If a pencil of parallel rays be incident on a convex spherical speculum, they will be reflected diverging; and the focus of the reflected rays will bisect the radius which is parallel to the incident rays, and be virtual. If a radiant object be placed before a convex speculum, 1. The image will appear behind the speculum. 2. It will
appear erect. 3. It will be less than the object. 4. As the object approaches the speculum, the image will likewise approach the speculum; and increase, till at length, when the object touches the speculum, the object and image will meet, and be equal. 5. The image will be convex towards the object.
If a pencil of parallel rays fall on a spherical concave speculum, they will be reflected converging; and the focus of the reflected rays will bisect the radius that is parallel to the incident rays, and be real. If rays diverging from a radiant placed in the centre, fall on a concave spherical speculum, the focus will coincide with the centre, and the image with the object. If the radiant be placed between the centre and the principal focus, the image will be formed at the other side of the centre; and the distance of the radiant from the speculum will be to its distance from the centre, as the distance of the focus from the speculum to its distance from the centre. If the radiant be placed in the principal focus, the rays will be reflected parallel. If the radiant be placed between the principal focus and the speculum, the rays will be reflected diverging from an imaginary focus behind the speculum ; and the position of the focus will be determined in the same inanner as before. The image is inverted or erect with respect to the object, according as they are at different sides,' or on the same side of the centre of the speculum. The degrees of heat generated in the foci of different speculums when exposed to the sun's rays, are as their areas directly, and inversely as the squares of their focal lengths. And the heat generated in the focus of a speculum employed as a burningglass, is to the sun's direct heat, as the area of the speeulum to the area of the image.
4. Vision. The eye consists of coats, muscles, vessels and humours. The three coats are, the scleroticu, the choroides, and the retina ; and the humours are called aqueous, crystalline, and vitreous. (1.) The aqueous humour is in all respects like water, but of a spirituous nature. (2.) The crystalline is transparent, and more solid than either; its figure resembles an optic lens, convex on both sides, and its use in the eye is the saine. (3.) The ritreous, or glassy, is like the white of an egg; it is in greater abundance than either of the other humours; it gives the eye its spherical form ; and is thicker than the aqueous,
but thinner than the crystalline humour. Next this bumour, on the bottom of the eye, is spread a fine membrane or net-work, called the retina, through which are expanded the medullary fibres of the optic nerve, which proceed from the brain. The rays of light, falling upon the aqueous humour of the eye, are through it refracted to the crystalline humour, by which, as a double convex lens (kept always at a proper distance hy the glassy humour) they are all converged-and painted in an inverted form upon the retina. The impression is, communicated to the common sensory of the brain by the optic nerves, and there presents to the mind the species and image of the object. The most extensive prospect is thus compressed into the compass of a sixpence, yet circumstantially represented. A stage coach, travelling at its ordinary rate, for half an hour, passes in the eye only over one treelfth part of un inch, yet the change of place is distinctly perceived throughout its whole progress.
5. Defective Sight. Some eyes are too flat, others are too convex; in some, the humours lose a part of their transparency, and on that account, much of the light that enters the eye, is stopt, and lost in the passage, every object having a dim or confused appearance. Spectacles, are intended to collect the light, or bring it to a greater degree of convergency. They are made convex when the eye is too flat (a defect common to old people,) and concave when the eye is already very convex, that is, when near objects are seen distiycily, and distaut ones are dis
cerned but feintly. Concave glasses cause the rays of light coming to the eye to diverge, and consequently prevent them from converging or coming to a focus, before they arrive at the retina. The disorder is called a caturact, when the crystalline humour loses its transparency, and the operation performed, couching. This is done by thrusting a fine awl through the coats of the eye, and pushing the crystalline to the bottom of the eye, where it remains. The deficiency may be supplied with a convex lens. When the optic nerve is affected, it is called a gutta serena, and the disorder is generally incurable.
6. Single Microscopes, of the greatest power, are very small globules of glass, which are made by melting the ends of fine threads of glass in the flame of a candle; or by taking a little fine pulverized glass on the point of a very small needle, and melting it into a globule. With such microscopes as these, Leewenhoek made all his won. derful discoveries. (1.) The compound microscope, consists of at least two lenses, by one of which an image is formed within the tube of the microscope ; and this image is viewed through the eye-glass, instead of the object itself, as in the single microscope. The microscope being intended only for minute objects, the object-lens is consequently of a shori focus, and the eye-glass, in this case, is not of so bigh a maguitying power as in the telescope. (2.) The solar microscope, is a kind of camera obscura, which, in a darkened chainber, throws the image on a wall or screen. It consists of two lenses fixed opposite a hole in a board or window-shutter. There is also a plain reflector placed without, movedl by a wheel and pinion, which may be so regulated as to throw the sun's rays upon the outer lens. (3.) The lucernal microscope, invented by Mr. Adams, is also a kind of camera obscura; only the light, in this latter case, proceeds from a lamp instead of from the sun, which re. ders ii convenient to be used at all times. See his Microscopical Essays, Svo.* The principle of the telescope, may be easily understood from what has been said on the nature of the compound microscope. Telescopes
* In the Philosophical Transactions for 1812, part II. p. 375, Dr. Wollaston describes a periscopic microscope which he has invented. and which seems to be very superior to those Row used.