6. We have supposed the image m n to be received on a smooth and white surface of paper, or other material, on which a picture of it is distinctly formed; but if we receive it upon ground glass, or upon a plate of glass one of whose sides is coated with a dry film of skimmed milk, and if we place our eye eight or ten inches behind this semi-transparent ground interposed at m n, we shall see the inverted image, m n, as distinctly as before. If we keep the eye in this position, and remove the semitransparent ground, we shall see an image in the air distinctly, and brighter than before. The cause of this will be readily understood when we consider that all the rays which form by their convergence the points mn of the image m n, cross one another at m n, and diverge from these points in the same manner as they would from a real object of the same size and brightness placed at m n. The image m n may therefore be regarded as a new object; and by placing another lens behind it, another image of the image m n would be formed, exactly of the same size and in the same position as it would have been had m n been a real object. But since this secondary image of m n must be inverted as regards the first image, it will be erect in regard to the object м N, so that by using one or more lenses we can obtain inverted or direct images of any object at pleasure.

7. The CAMERA OBSCURA is nothing but a wooden box made to imitate a darkened chamber, with a proper lens or glass to concentrate the pencils of light, and make

the picture more vivid and distinct. The glass is fixed on a sliding frame, which enables it to be drawn out to the proper length for making the picture perfect, and a piece of mirror is fixed at the opposite end of the box, at an angle of 45, to throw up the picture on a piece of ground glass which is let into the top of the box,

Fig. 34.

with a hinged wooden cover with wooden sides to it, alike to protect it and to shut out the light from outside. Fig. 34 shows the common box camera. The dotted lines mark the position of the mirror.

BURNING-GLASSES.'

1. The use of the laws of the reflection and refraction of light to concentrate the sun's heat on a given object, so as to set it on fire, has been

154

long known. It is probable that the ancient Romans and Druids had a method of lighting their sacred fire by means of concave reflecting metal mirrors, and Archimedes is said to have burned the Roman fleet by this means. A burning-glass was presented to the Emperor of China when Lord Macartney visited Pekin, which melted a crystal pebble in 6 seconds, a piece of white agate in 30 seconds, and a cornelian in 75 seconds. It was 3 feet in diameter, and weighed 212 lbs. Its focal distance was 6 feet 8 inches, but this was generally shortened by a smaller lens.

2. The rays of the sun may also be brought to a point or focus by reflection from a concave mirror, or from a combination of mirrors. By a contrivance composed of 108 small flat mirrors, so arranged that they all reflected heat to the same spot, Buffon, the French naturalist, was able to set wood on fire at the distance of 209 feet, and to melt lead at 100 feet,

and silver at 50 feet. This is illustrated in Fig. 35, in which two concave mirrors are placed exactly opposite each other. If a hot body be placed at c, in the focus of the mirror A, all the rays which it sends to that mirror will be reflected in parallel lines, and will reach the other mirror thus, and be reflected by it so as to meet in the focus D. A red-hot iron ball placed in the focus of A will in this way set fire to any combustible, such as paper, gunpowder, &c., placed in the focus of B, though at a distance of 10 or 15 feet. From the same laws, a glass globe filled with water gathers the sun's rays into a focus, and hence it is dangerous to leave a gold-fish globe, or a decanter of water, near a window, in the full light of the sun, if the weather be bright and hot.

R

Fig. 36.

3. The principle by which a burning-glass produces such effects is, that it concentrates on a point, or focus, the heat as well as brightness of the rays of the

sun.

Thus, when converging rays, or rays which proceed to one point, such as R L, R L (Fig. 36), are intercepted by, or fall upon, a convex lens L L, they will be refracted so as to converge to a point or focus f, nearer the lens than its principal focus o. As the point of convergence F recedes from the lens, the point ƒ also recedes from it towards o, beyond

which it never goes; and as r approaches the lens, ƒ also approaches to it. The points F and ƒ are called conjugate foci, because the place of one varies with the place of the other, and, though every lens has but one principal focus, yet its conjugate foci are innumerable.

4. When diverging rays, or rays which proceed from one point, such as F (Fig. 37), fall upon a convex lens L L, whose principal focus is o, the refraction of the lens will

cause them to converge to a focus f, beyond o. As the point F recedes from the lens, the focus ƒ will approach to it, and when F is infinitely distant, as

I

Fig. 37.

the sun may be said to be, or when the rays are parallel, ƒ will coincide with o. If F approaches to o, the focus ƒ will recede from o; and when F coincides with o, ƒ will be infinitely distant, and the refracted rays will be parallel. When F is between o and c, the refracted rays will diverge. The points F and ƒ are called conjugate foci, as in the case of converging

rays.

THE MAGNIFYING-GLASS.

1. In order to explain how lenses increase or magnify objects, and make them appear as if they were brought nearer to us, the reader must understand what is meant by the apparent magnitude of objects. If an eye placed at E looks at a man, a b (Fig. 38), placed at a distance, his general outline only will be seen, and neither his age, nor features, nor his dress will be distinguished. When he is brought gradually nearer to us we discover the separate A

parts of his dress, till,
at the distance of a few
yards, we perceive his
features; and when he
is brought still nearer 6
we can observe the finest
lines in his skin. At the

B

Fig. 38.

distance Eb the man is seen under the angle bE a, and at the distance E B he is seen under the greater angle B E A or b E A', and his apparent magnitudes, a b, A′ b, are measured in those different positions by the angles b E α, B E A, or b E A'. The apparent magnitude of the smallest object may therefore be equal to the apparent magnitude of the greatest. 2. Let us now suppose the man a b to be placed at the distance of 100 feet from the eye at E, and that we place a convex glass of 25 feet focal

distance half-way between the man a b and the eye, that is, 50 feet from each, then an inverted image of the man will be formed 50 feet behind the lens, of the same size as the man, say 6 feet high. If this image is looked at by the eye placed 6 or 8 inches behind it, it will be seen quite distinct, and nearly as well as if the man had been brought from the distance of 100 feet to the distance of 8 inches from the eye, at which the details of his personal appearance can be examined. Now, in this case the man, though not actually magnified, has been apparently magnified, because his apparent magnitude has been increased in the proportion of 8 inches to 100 feet, or 150 times.

3. But if instead of a lens of 25 feet focal length we make use of a lens of a shorter focus, and place it in such a position between the eye and the man that its conjugate foci may be at the distance of 20 feet, and 80 feet from the lens, that is, that the man is 20 feet before the lens, and the image 80 feet behind it, then the size of the image is four times that of the object, and the eye placed 8 inches behind this magnified image will see it with great distinctness. In this case the image is magnified 4 times directly by the lens, and 150 times by being brought 150 times nearer the eye, so that its apparent magnitude will be 600 times larger than before.

4. If we use a lens of a still smaller focal length, and place it in such a position between the eye and the man that its conjugate foci may be at the distance of 75 feet, and 25 feet from the lens, that is, that the man is 75 feet before the lens, and his image 25 feet behind it, then the size of the image will be only one-third of the size of the man; but though the image is thus diminished three times in size, yet its apparent magnitude is increased 150 times by being brought within 8 inches of the eye, so that it is still magnified, or its apparent magnitude is increased 150, or 50 times.

MICROSCOPES.

1. A microscope is an optical instrument for magnifying and examining minute objects. Microscopes may be divided into two classes, viz., simple and compound. The simple microscope, said to have been separately invented by Jansen and Drebell, is nothing more than a lens or sphere of any transparent substance, in the focus of which minute objects are placed for examination. The rays of light which proceed from each point of the object are refracted by the lens into parallel rays, which, on entering the eye, placed immediately behind the lens, affords distinct vision of the object.

The principle of the microscope has been already explained in describing the magnifying-glass, so that it is not necessary to repeat it. 2. Microscopes may be formed in a very simple manner, by inserting

drops of clear water in small apertures in a sheet of tin, or any thin solid substance. Sir David Brewster made them in this way with oils and varnishes; but the finest of all simple microscopes may be formed, he states, by forming minute plano-convex* lenses, upon glass, with different fluids. The spherical crystalline lenses of the eyes of minnows and other small fish form excellent microscopes, taking care that the observer looks through the lens in the same direction that the fish did.

A simple microscope, of very convenient form, consisting of a single lens, is shown in Fig. 39. This form of lens was invented by Sir David Brewster, although it has received the name of the Coddington lens, from its supposed invention by the mathematician of that name.

3. It is formed by cutting an angular groove round a solid globe of glass about half an inch in diameter, leaving two spherical surfaces, A B and CD, on opposite sides, uncut. The angular groove, a E C, B FD, is then filled up with opaque matter, the circular edge of the groove E F serving as a diaphragm between the two spherical surfaces. It is evident from the figure that the effect of the lens upon the rays o o will be the A same wherever the point o may be situated; the lens, therefore, gives a large field equally well defined in all directions, and is very convenient as a hand and pocket glass. Sir David Brewster states, that when this lens is formed of garnet, and used in homogeneous† light, it is the most perfect of all lenses, either for single microscopes or for the object lenses of compound ones.

4. In its most simple form the compound microscope is composed of a magnifying lens or combination of Fig. 39. lenses, by which an enlarged image of a minute object is produced, and another magnifying lens, or combination of magnifying lenses, by which such image is viewed as an object would be by a simple microscope.

The former is called the object-glass, or objective, as it is always immediately directed towards the object, which is placed very near to it; and the latter the eye-glass, or eye-piece, as the eye of the observer is applied to it to magnify the image of the object.

5. A combination of lenses forming a compound microscope is shown without the mounting, in Fig. 40, where o is the object, L the objectglass, and E the eye-glass. The object-glass is of very short focal length, and the object o is placed in its axis a little beyond its focus. An inverted image o o, of the object o, will be produced at a distance from the objectglass, L, much greater than the distance of o from it.

The observer will adjust the eye-glass E at such a distance from the image as will enable him to see it most distinctly. To estimate the entire

* With a flat and a convex side.

+ Of one kind.