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surface we might suppose the plane face of a prism. Also the other half, as P'AB, is equivalent to a number of prisms all turned the other way; so that the whole effect of a double-convex lens, as PP', is to cause a double refraction of all rays that are transmitted through it inwards. Thus, suppose a number of parallel rays to fall on the lens PP'; let one fall at P; then at the first surface it is refracted into the direction Pq, and if not further interfered with, it would meet the axis at q (corresponding to F in fig. 30), but at the second surface it is again refracted, and thrown more towards the thick part of the lens, so that it meets the axis at F. Again, of those that fall on the other side, let one fall at P'; it is refracted at the first surface into the direction P'q, and then at the second it is also turned to F. This is what takes place in the eye, when the light that enters by the pupil is transmitted through the crystalline lens. (HUMAN PHYSIOLOGY, page 74.) The point, F, at which the rays of light meet the axis or middle line O'q (for they all meet it at one point), is called the focus. In the eye this focus falls on the retina, and the cause of defective eyesight is simply that the rays of light are brought to a focus, not on the retina, but either in front of or behind it. In the former case, the individual is said to be short-sighted or near-sighted, and in the latter, far-sighted. The principle on which spectacles help to remedy those defects will be explained presently. After what has been said of a convex surface and the convex lens, it needs no proof to shew that the effect of a concave surface and of a double-concave lens (fig. 29) is exactly the reverse. As the normals to a concave surface all meet in the centre of the curve outside the body, inside they all diverge; and therefore parallel rays of light transmitted through a medium with a surface meeting them, being all refracted towards the normals, must all be made to diverge also. Then as to a double-concave lens, if we suppose it to be the same as two prisms with their thick part outwards, it is at once clear that all rays transmitted through it must suffer a double refraction outwards, that is, be made to diverge.
A very few words will now make the principle of spectacles perfectly intelligible. In the case of a near-sighted person, the defect in his sight is that rays of light are brought to a focus in front of the retina, the cornea and crystalline lens making the rays converge too much. To remedy this, it is necessary to make the rays diverge a little before entering the eye. This we saw to be done by a double-concave lens; therefore, near-sighted persons often wear spectacles with double-concave lenses. The defect in the case of far-sighted persons is that light is brought to a focus behind the retina : the refracting power of the cornea and crystalline lens is not strong enough, and it is necessary to make the rays converge. This, as we saw, is the effect of a convex lens ; therefore, far-sighted people wear spectacles with double-convex lenses.
ELECTRICITY received its name from the Greek word elektron, amber, in which substance it was first detected. It was found, from the earliest times, that when a piece of amber was rubbed with a dry cloth, it had the power of attracting small light bodies lying near it. In later times, it was observed by scientific men that sulphur, glass, sealing-wax, and many other substances had the same property; and now-a-days electric experiments are all made with these common substances, instead of the comparatively rare and costly amber, especially since they exhibit the phenomena of electricity in as great perfection as the substance in which it was first discovered.
In order to observe better what takes place, let us suspend something very light—a little ball of pith of the elder-tree is generally used —
by a silk thread from a glass tube, as in the
first be attracted, and then repelled. If the Fig. 32.
glass be rubbed again, the same thing may be
repeated of course ; so with the wax; and the pith-ball might thus be kept playing between the two for any length of time. There are thus two kinds of electricity, one produced in glass when rubbed with a piece of silk ; and the other in sealing-wax when rubbed with a woollen cloth. When the electrified glass attracts the pith-ball, electricity is communicated to the ball, and then it is repelled ; in other words, two bodies charged with the same kind of electricity repel each other. Then the ball is attracted by the wax, and is only repelled again when the electricity received from the glass has been replaced by the kind produced in the wax ; and we infer from this that two bodies charged with the different kinds of electricity attract each other. Special names have been given to these two kinds of electricity : the kind produced in glass and a number of other substances is called vitreous, from Latin vitrum, glass ; while that produced in sealing-wax and a number of other substances of a resinous nature is called resinous. Instead of these two names, however, positive and negative are now more frequently used ; and for convenience, the algebraic signs corresponding to these are employed, + and — The sum of what has been said may now be stated thus : Bodies charged with either positive or negative electricity attract bodies charged with negative and positive electricity respectively, and attract bodies not electrified at all; but they repel all bodies charged with electricity of the same kind as their own; further, electricity can be communicated by contact from one body to another.
In the above experiment, a particular substance is rubbed with a particular kind of cloth, the glass with silk, and the sealing-wax with a woollen cloth. When electricity is produced in a substance by friction, electricity of the opposite kind is produced in the rubber or substance (a cloth generally) with which it is rubbed ; and a body, which becomes charged with positive electricity when rubbed with one substance, may become charged with negative when rubbed with another. Thus, when glass is rubbed with silk, the electricity of the former is positive, and that of the latter, negative ; but if the glass be rubbed with a cat's fur instead of silk, the electricity of the former is negative, and that of the latter, positive. In the following list, each body, when rubbed with the one before it in order, becomes charged with negative electricity ; when with the one coming after it, with positive—cat's fur, glass, linen, feathers, wood, paper, silk, shell-lac, ground glass.
It must be remembered that all bodies do not become charged with electricity when rubbed; those that do are called electrics, and those that do not, non-electrics. There is another distinction to be noticed in bodies, and that is with regard to receiving the electricity which has been produced in an electric; those that do are called conductors, and those that do not, non-conductors. Most bodies are electrics, but it makes a material difference in this respect whether the body be a conductor or not; because, if the electric be also a conductor, the electricity produced is immediately carried off; while, if it be a non-conductor, the electricity remains on the surface, and is more apparent. Thus it happens that practically the most powerful electrics are non-conductors. The most important class of conductors are the metals. Water is also a good conductor. A few of the principal non-conductors, and thus, in a certain sense, the best electrics, are shell-lac, caoutchouc, amber, resin, sulphur, wax, glass, gems, silk, wool, hair, dry paper, leather, camphor, chalk, lime.
We can now explain why the pith-ball, made use of in the experiment, was suspended by a silk thread from a glass stand. Glass and silk are non-conductors, and so the electricity communicated to the pith-ball could not be conveyed away by these substances, as it would have been if it had been attached to a conductor ; as, for instance, if it had been fastened by a
wire to an iron stand. A conductor cut off from communication with the ground by being fixed on non-conductors, in this way, is said to be insulated.
Let us now return to our first experiment. We saw by it that positive electricity attracted negative, and negative, positive ; and also that both attracted bodies not electrified at all : it will now be shewn that the last part of this statement is the same as the first. If a brass cylinder be fixed on a glass stand, with a pair of pith-balls suspended by a cotton thread (not silk this time, because it is not desired to insulate them from the cylinder) at either end, and placed near another insulated conductor charged with positive electricity, this is what will happen—the balls will
be repelled from each other, as in the figure, shewing that each pair has become charged with the same kind of electricity. But it is found that the electricity at the two ends is of different kinds, that in the end next the ball being negative, while that in the other end is positive.
This is quite in accordance with the result of Fig. 33.
our first experiment, that positive electricity
attracts negative, and repels positive, and vice verså. From this it is manifest, that in all conductors there exist (without friction) the two kinds of electricity, and that these are acted upon by an electrified conductor exactly in the same way as if the electricity had been produced by friction in both. It will now be seen that, when the rubbed glass attracts the pith-ball the first time, the attraction is not different from the subsequent attractions; the positive electricity in the glass repels the positive in the ball to the opposite side, and attracts the negative. This attraction is great enough to overcome the slight weight of the ball ; when contact takes place, the negative electricity in the side of the ball next the glass is replaced by positive electricity from the glass. The ball is then charged altogether with positive electricity, and of course is now repelled by the glass ; but at the other side, the sealing-wax which is charged with negative electricity attracts it; and so on. The influence which an electrified conductor thus exercises on a non-electrified one is called Induction.
Lightning In the case of the pith-ball, when induction takes place, owing to its lightness and the manner in which it is suspended, it comes in contact with the glass, and electricity thus passes to it from the glass ; but if, in fig. 33, there were a sufficient quantity of electricity in the ball, and the attraction were great enough, it would pass to the cylinder without contact; and there would be a slight flash of fire and a slight crack. Now, this flash
and this crack are lightning and thunder in miniature, for the black thunderclouds that we see in a thunder-storm are clouds charged with electricity. When two thunder-clouds approach each other, induction takes place, and owing to the immense quantity of electricity the two contain, when it passes from the one to the other, the flash is of exceeding brightness, and the report deafening. Sometimes the induction takes place between a cloud and a prominent object on the ground, such as a steeple, a tall chimney, a tree, or an animal. Anything will do, if it form a point near enough the cloud to cause the electricity to overcome the resistance of the intermediate space; and the consequences are generally fatal to the object in this position. But it is not necessarily so; the destructive effects of lightning, or of electricity rather, only take place when it meets with resistance that is, when induction takes place between a thunder-cloud and some object which is a bad conductor of electricity. Hence the object of lightning-conductors. Metals being good conductors, if a rod of metal have its point extending beyond the top of any prominent object, and pass down into the ground, when a thunder-cloud happens to be near, and induction takes place, the electricity, which would otherwise have shivered the non-conducting steeple or chimney, passes harmlessly down the metal rod.
It may now be asked where this vast quantity of electricity in the thunder-cloud comes from. We saw that electricity is produced by friction ; this, in the case of the thunder-cloud, is one source of it, for there are many different kinds of friction going on in nature. The friction produced by the wind in various ways is very great; thus electricity may be produced in a piece of glass by a blast of air from a bellows, instead of by rubbing. But, besides friction and all other forms of mechanical action, electricity is produced by all kinds of physical and chemical action; it is produced by the great system of evaporation which is continually going on from all bodies of water, and also from the processes of vegetation in which water is being continually separated and evaporated from plants, so that the vapours and gases that rise into the air are all
or less charged with electricity; and this electricity, which is generally diffused through the atmosphere, becomes occasionally concentrated in clouds, and is then liberated in the form of lightning.
The Electric Telegraph. In the form of lightning, electricity is often very destructive, and is at all times very terrible; but the reason and skill of man have enabled him to turn it to account in a most useful manner. By means of the electric telegraph, a message can now be sent half round the world in a few hours.
We have said that one of the sources of electricity is chemical action;