same relative parts of the retina, the nerve is exhausted; but when the eye shifts, there is a new exercise of the nerve. The part of the retina that was opposed to the lights is now opposed to the shades, and what was opposed to the different colours is now opposed to the other colours, and the variation in the exciting cause produces a renewed sensation. From this it appears how essential the incessant searching motion of the eye is to the continued exercise of the organ.

The familiar fact which we have to carry with us into this inquiry, is, that if we throw a silver coin upon a dark table, and fix the eye upon the centre of the coin, when we remove the coin there is, for a moment, a white spot in its place, which presently becomes deep black. If we put a red wafer upon a sheet of white paper, and look upon it, and continue to keep the eye fixed upon the same point, upon removing the wafer, the spot where it lay on the white paper will appear green. If we look upon a green wafer in the same manner and remove it, the spot will be red; if upon blue or indigo the paper will seem yellow. These phenomena are to be explained by considering that the nerve is exhausted by the continuance of the impression, and becomes more apt to receive sensation from an opposite colour. All the colours of the prism come into the eye together from the surface of the white paper when the wafer is removed; but if the nerve has been exhausted by the incidence of the red rays upon it, it will be insensible to these red rays when they are thus reflected together with the others from the white paper; the effect of the rays of an opposite kind will be increased, and, consequently, the spot will be no longer white, but of the prevailing green

colour.

Let us see how the loss of sensibility produces an effect in engraving, where there is no colour, and only light and shade.

Is it possible that a high tower, in a cloudless sky, can be less illuminated at the top than at the bottom? Yet, if we turn to a book of engravings, where an old

steeple or tower is represented standing up against the clear sky, we shall find that all the higher part is dark, and that the effect is picturesque and pleasing. Now, this is perfectly correct, for though the highest part of the tower be in the brightest illumination, it is not

The reason is

seen so; it never appears so to the eye. that when we look to the steeple, a great part of the retina is opposed to the light of the sky; and on shifting the eye to look at the particular parts of the steeple, the reflected light from that object falls upon the retina, where it is exhausted by the direct light from the sky. If we look to the top of the tower, and then drop the eye on some of the lower architectural ornaments, the effect infallibly is, that the upper half of the tower is dark. For example, if looking to the point A, fig. 2, we drop the eye to B, the tower from A to B is seen by that part of the retina which was opposed to the clear sky from A to C; and it is dark, not by contrast, as it would be thoughtlessly said, but by the nerve being somewhat exhausted of its sensibility.

ELECTRICITY.

ELECTRICITY is the term employed to designate that important branch of experimental philosophy which relates to the properties exhibited by certain substances when rubbed against, or by some other means made to communicate with, each other. It is derived from electron, the Greek word for amber, electric phenomena having been first observed in that body, when rubbed against flannel, or on a coat sleeve.

Of the true nature of electricity we are compelled to acknowledge our ignorance. There is no doubt that it pervades all material bodies, animate as well as inanimate, but in what it consists or how it is constituted are questions too difficult for us to solve. We do not even know whether electricity is material or not. If it be, it is so subtle and refined in its nature that it passes with inconceivable velocity through the hardest substances, and if allowed to accumulate in them, it does so without making any difference either in their weight or their dimensions. On this account it is that electricity (as well as light and heat) is denominated an imponderable element, to distinguish it from those forms of matter which possess the qualities of length, breadth, and thickness, and consequently weight.

Electricity is developed in a variety of ways; but whatever be the nature of the materials, or of the process employed, we may justly conclude that the principle is in all cases identical, however different it may appear to be either in its effects or its mode of operation.

When a piece of glass is rubbed with silk, or a stick of red sealing-wax with woollen cloth, each substance acquires a property not possessed by it whilst in a quiescent state, and which consists in alternately attracting and repelling feathers, straws, dry leaves, fibres of cotton, and many other light substances. The electricity thus excited is called ordinary, and sometimes. common electricity.

The following simple apparatus will illustrate quite sufficiently for our purpose electrical excitement:

Let a clean and very light downy feather be attached to a piece of white sewing silk about three feet long, and suspended from the ceiling or other part of a room in such a manner, that it shall be eighteen inches or two feet distant from all surrounding bodies; then provide a piece of glass tube, say, three-fourths of an inch in diameter, and thirty inches long, the tube being perfectly clean and dry. If it be rubbed briskly with a warm and dry silk handkerchief, it will be electrically excited, and on advancing it slowly towards the feather, the latter will be attracted by and adhere to it; but on separating them, and again bringing the tube near the feather, that body will be as promptly repelled as it was before attracted. After a little time the feather will again approach the tube and again be repelled by it, and this alternate action will continue until the whole of the electricity excited on the surface of the tube has been dissipated; but a fresh supply may be obtained, as often as required, by rubbing the tube with the handkerchief, as already described.

One of the most important principles connected with the science of electricity is indicated by the preceding experiment, which is, that there are two kinds, or, if not two kinds, two opposite states of electricity. Thus, when the feather has received a portion of the electricity which is excited by friction on the glass, it is no longer attracted by the latter, but, on the contrary, repelled; whence it is inferred that the electricity of the feather whilst in a quiescent state, and that of the glass after being rubbed with silk, are dissimilar, and therefore it is concluded that bodies imbued with opposite kinds, or which are in opposite states of electricity, attract, and those in similar states repel each other.

The distinction to which we have just referred will be more satisfactorily shewn, if we take a large stick of red sealing-wax and excite it by rubbing it with a piece of dry and warm woollen cloth. On presenting the excited wax to the feather it will be first attracted, then repelled,

as noticed with the glass; but when the feather is repelled by the wax, if we approach it with the excited glass it will be instantly attracted, and when repelled by the glass it will be attracted by the wax. It is hence sufficiently plain that the electricity developed by glass differs from that produced by wax; and whether the difference is described as being dependent on opposite kinds, or opposite states of electricity, the effect is the same.

If we take a piece of glass tube, rub it with a silk handkerchief, and hang it to a piece of string, then rub a piece of sealing-wax with a bit of woollen cloth, and present one end of it to one end of the glass tube as it hangs, the wax will attract the glass towards it. On the contrary, if we rub a piece of glass and present it to one end of the suspended glass, we shall find it drive the latter away from it, or, as we say, repel it.

So, if we suspend a piece of sealing-wax in the same way, after rubbing it, and then put a piece of rubbed sealing-wax to one end of it (so as just not to touch it), we shall find it repel the other, while a piece of rubbed glass will attract the wax.

We might use many other substances instead of wax, such as sulphur, amber, shellac, gutta percha, resin (or what are called resinous substances); and, instead of glass, we might use crystal, diamond.: or other precious stones of a glassy kind (or what are called vitreous substances), and we should find the general law,—that resinous substances when rubbed repel each other, and vitreous also repel each other, but that resinous attract vitreous bodies, and vitreous attract resinous; or, in short, that like repels like, and opposite attracts the opposite. This, then, is the fundamental law of electric action, electricities of the same nature repel each other, electricities of opposite natures attract each other.

The electricity on glass used to be called vitreous, and that on wax resinous, but now the former is called positive and the latter negative; so the feather, when charged with electricity from the glass, is said to be positively, and when charged from the wax, negatively