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If an object be placed before a concave mirror at a distance from it greater than that of the geometric centre of its curvature, an image will be formed of this object in front of the mirror at a distance from its surface greater than half its radius.
This image will be inverted, and will be less than the object; as the object approaches the centre of the curvature of the mirror, the image will also approach that point, and thus the object and image will approach each other; the image will at the same time be increased in magnitude. If the object be placed within the centre of curvature of the mirror, but farther from its surface than half its radius, a magnified image will be formed at a distance more or less considerable in front of the mirror. Thus, let us suppose that a mirror formed with a curvature having a radius of four feet, has an object in front of it at a distance of three feet from its surface an image of that object will be formed at six feet in front of the mirror, and this image will be double the height or length of the object.
In this manner, a mirror placed out of sight of a person may be made to throw the image of an object close to him; thus a dagger may be presented to one's bosom, which, however, is literally an air-drawn dagger.
The only form of reflecting surface which presents an object in its natural position and proportions is the plane mirror commonly used for domestic purposes; and even this, as already explained, reverses the object laterally-making right left, and left right. For the purposes, however, to which it is usually applied, this derangement does not impair its utility.
The perfection with which a mirror presents the image of an object placed before it depends upon its form and material. It is, above all things, essential that its surface should be perfectly plain and even; any deficiency in this quality will produce a corresponding distortion of the image. Cheap lookingglasses are often striated and streaked with inequalities and ridges, which render them nearly useless. Whatever be the substance used to form a mirror, a part only of the light which falls upon it will be instrumental in forming the image. The entire quantity of light which falls on the mirror may be accounted for as follows:
1. A part will be regularly reflected according to the laws above explained and it is by this part the image will be formed.
2. Another part will be irregularly reflected—that is to say, it will be scattered in every direction around from every part of the surface. It is this portion of the light which renders the surface of the mirror visible.
3. A part will be absorbed upon the reflecting surface and lost. The more highly polished and even the reflecting surface is, the less will be the part irregularly reflected, and the brighter will be the image. The part of the light absorbed or stopped will depend on the physical quality of the matter of which the reflector is formed.
Since art cannot produce a perfect reflecting surface, there will always be a portion of the incident light irregularly reflected and absorbed. It follows, therefore, that light is always lost in reflection; and in the case of plane mirrors, where the magnitude of the image is equal to that of the object, the brightness of the image must always be less than that of the object.
There is no substance which reflects with equal facility all tints of color. It generally happens that lights of one tint are more absorbed than the lights of another. Mirrors, therefore, will produce a change more or less according to their degree of imperfection in the tints which characterize the object before them; in other words, the color or tints of the image will not correspond exactly with those of the object.
It is therefore a fact true in science, although sometimes ridiculed, that
different looking-glasses will present a more or less agreeable representation of the person who uses them, according to the colors which they may happen to absorb. Thus, if a mirror has a tendency to absorb the red tints, it will give a pallid tint to the complexion; whereas, if it absorb the blue tints, it will throw a blush over the appearance, and may be called a flattering glass.
Glass is the most convenient material for mirrors intended for domestic use, because it is the cheapest and most durable; but it is far from being the best. Its defects will become apparent by considering the mode in which its effects are produced. A coating of metallic foil is attached to the hinder surface of the glass, and by the mode of its adhesion a smooth metallic surface is thus formed under or behind the glass. It is this surface, and not the front of the glass, which is the real mirror: it is by it that the images of objects in front of the looking-glass are produced. The light has to pass through the thickness of the glass to reach this surface, and after being reflected by it, has again to pass through its thickness in order to reach the eye and produce a perception of the image. There are here three successive stages in which light is lost. A part only of the light which strikes upon the front surface of the glass penetrates it, and a part of what does penetrate it is lost upon the hinder surface; and again, after reflection, in issuing through the front surface, another portion is lost.
But the loss of light is not the only defect: in passing through the glass, partial absorption of color takes place; and hence, as has been already stated, the tints of the image will be different from those of the object.
A portion of the light which falls on the front surface of the glass is regularly reflected, and produces a faint image of the object, which, by careful observation, may be easily distinguished a little in front of the stronger image. produced by the silvered surface. The distance of this faint image in front of the other will be equal to the thickness of the glass.
It is evident, from what has been just observed, that the thinner the glass is, the better will be the mirror.
The defects which have been just explained have rendered glass reflectors inapplicable to telescopes or any of the class of superior optical instruments used for scientific purposes. In these instruments metallic reflectors alone are used. An alloy of metals is selected for this purpose as white as possible in color, and susceptible of a high polish. A very accurate figure is imparted to it and a very perfect polish by various processes known in the arts. Although with such reflectors incomparably less light is lost than in common looking-glasses, still a much greater loss of light takes place than in transmission through transparent media; hence the received maxim in optics, that more light is lost in reflection than in refraction. Liquid surfaces afford in general, when at rest, good plane reflectors. If the liquid be opaque, the reflection is very perfect. This will be rendered apparent by pouring some clear quicksilver on a plate; to exhibit this effect, the quicksilver should be strained through a piece of chamois leather: it would otherwise have a film upon it composed of foreign matter, which would destroy its reflecting
The objects on the banks of a calm river or a tranquil lake will be seen reflected in its surface; but it is worthy of notice that the observer can only see this reflection when he looks very obliquely at the surface of the water: the reason of which is, that the rays which strike nearly at right angles to the water penetrate it in virtue of its transparency. It is only those which glance obliquely on it that are reflected; just as a stone which, thrown perpendicularly on the water, would immediately sink, will, if projected at a
PROSPECTS OF STEAM-NAVIGATION.
Retrospect of Atlantic Steamers.-Origin of the Great Western.-Cunard Steamers.-Can Steam Packet-Ships be Successful?-Difficulties attending their Operation.-In a commercial Sense.In a mechanical and nautical Sense.-Great Expedition must be given up.-Defects of Common Paddle-Wheels-Defects of the present Steam-Vessels as applicable to War.-Difficulty of long Ocean-Voyages.-Ericsson's Propeller.-Loper's Propeller.-Advantages of Submerged Propeller.-Method of raising the Propeller out of the Water.-Fuel.-Form and Arrangement of the proposed Steam Packet-Ships.-War Steamers.-The Princeton.-Effects to ensue from the New Steamships.-Conclusion.