celestial bodies, being refracted by the atmosphere? 13. What occasions twilight? 14. How would the heavens appear if there were no atmosphere? 15. Illustrate the reflection of light by fig. 29. Engr. III. 16. Refraction of light by fig. 29. [NOTE. Fig. 31. is a vessel with a flower in water at the bottom, seen by the eye in the direction of the rays which enter it. This experiment, and many others, may be easily performed.]

LESSON 32.

Different kinds of Lenses.

Diverge', rays of light coming from a point, and continually separating as they proceed, are said to diverge; the point is called the radiant point.

Converge', rays which tend to a common point are said to converge.

A Beam of light is a body of parallel rays; a Pencil of rays is a body of diverging or converging rays.

Cam'era obscu'ra, a chamber darkened; an optical machine used in a darkened chamber.

If

A LENS is a glass ground into such a form as to collect or disperse the rays of light which pass through it. They are of different shapes, from which they take their names. rays proceed from a radiant point distant as far as the sun, their divergency is so trifling that they may be considered as parallel. When parallel rays fall on a piece of glass having a double convert surface, that ray only, which falls in the direction of the axis of the lens, is perpendicular to the surface; the other rays falling obliquely, are refracted towards the axis, and they will meet beyond the lens at a point called its focus. The distance of the focus from the centre of the lens depends both upon the form of the lens, and upon the refractive power of the substance of which it is made; in a glass lens, both sides of which are equally convex, the focus is situated nearly at the centre of the sphere of which the surface of the lens forms a portion; it is at the distance, therefore, of half the diameter of the sphere. The property of a lens which has a double concave surface is to disperse the rays of light. Instead of converging towards the ray, which falls on the axis of the lens, they will be attracted towards its thick edges, both on entering and quitting it, and will, therefore, be made to diverge. Lenses which have

one side flat and the other convex or concave are less powerful in their refractions, than those which have been described. They are called plano-convex and plano-concave. The focus of the former is at the distance of the diameter of a sphere, of which the convex surface of the lens forms a portion. The last kind of lens is called a menis'cus, being convex on one side and concave on the other, like the glass or crystal of a watch.

All the parallel rays of the sun which pass through a convex glass are collected in its focus, and the force of the heat there is to the common heat of the sun, as the surface of the glass is to the surface of the focus. If a lens four inches in diameter collect the sun's rays into a focus at the distance of twelve inches, the image will not be more than one tenth of an inch in diameter: the surface of this little circle is one thousand six hundred times less than the surface of the lens, and consequently the heat will be one thousand six hundred times greater at the focus than at the lens. A globular decanter of water acts as a double convex lens, and furniture has been set on fire by leaving one incautiously exposed to the rays of the sun. A gentleman of London formed a burning-glass three feet in diameter, and when fixed in its frame, it exposed a clear surface of more than two feet eight inches in diameter, and its focus, by means of another lens, was reduced to a diameter of half an inch. The heat produced by this was so great that iron plates were melted in a few seconds; tiles and slates became red-hot in a moment, and were vitrified, or changed into glass; sulphur, pitch, and other resinous bodies, were melted under water; gold was rendered fluid in a few seconds. But notwithstanding this intense heat at the focus, the finger might, without the smallest injury, be placed in the cone of rays within an inch of the focus. On bringing the finger nearer, a sensation was felt like that produced by a sharp lancet, and not at all similar to the pain occasioned by the heat of fire or a candle. Substances of a white colour were difficult to be acted upon. Pure water in a clear glass decanter will not be warmed by the most powerful lens, but a piece of wood placed in the water may be burned to a coal. If a cavity be made in a piece of charcoal, and the substance to be acted on be put in it, the effect produced by the lens will be much increased. Any metal thus enclosed melts in a

moment; the fire sparkling like that of a forge to which the blast of a bellows is applied.

The image of an object when received through a convex lens will be inverted. If you cause the rays of light from the flame of a candle to pass through the glass of a common spectacle, and receive them on a sheet of paper, or dark skreen placed at a proper distance, you will see a complete inverted image of the candle on it. A convex lens placed in the hole of a window-shutter will exhibit, on a white sheet of paper situated in the focus of the glass, all the objects on the outside, as fields, trees, men, and houses, in an inverted order. The room should be quite dark, and the sun should shine upon the objects. A portable camera obscura may be made with a square box, in one side of which is to be fixed a tube, having a convex lens in it: within the box is a plane mirror, reclining backwards from the tube, in an angle of forty-five degrees. The picture is formed on a square of unpolished glass at the top of the box. If a piece of oiled paper be stretched on the glass, a landscape may be easily copied; or the outline may be sketched on the rough surface of the glass.

QUESTIONS.-1. What is a lens?-its axis?-focus? 2. Describe the five kinds of lenses. 3. What proportion is there between the common heat of the sun and the heat of the focus of a double convex lens? 4. Describe the burning glass formed at London. 5. What examples are given of images of objects being inverted by a convex lens? 6. How may a camera obscura be made? 7. Why is the mirror placed at an angle of 45 degrees exactly? Ans. To throw the image on the top, for incident rays, falling upon a surface declining 45 degrees, will be reflected at an equal angle of 45 degrees. 8. Describe figures 30. 36. 32. 33.

LESSON 33.

Mirrors.

Panoram'ic, exhibiting a succession of objects.

Opti'cian, a maker of optical instruments, one skilled in optics. MIRRORS are made of glass, silvered on one side, or of some metal highly polished. There are three kinds of them, the plane, the convex, and the concave. Objects seen in convex mirrors are diminished. A globe of glass, silvered

on the inside, is sometimes suspended from the ceiling of a room. It affords a sort of panoramic view of surrounding objects, though not all in natural proportion of size. When a convex mirror can be placed before a window, either with a good prospect, or where there are a number of persons passing and repassing in their different employments, the images reflected from it will be erect, and behind the surface; and a landscape or a busy scene delineated on one of them is always a beautiful object to the eye. Concave mirrors make objects appear larger, but distorted. If one be hung on the wall of a room, or fixed in a chair, a person beyond the focus sees his image inverted. As he puts forward his hand the image in the glass appears to do the same, as if to shake hands. As he tries to clasp the hand it vanishes from his view. Let the spectator hold out a knife in his hand, the image will appear to do the same; and so strong will be the impression on his mind, that he will feel a reluctance to run his hand forward against the apparent weapon. A concave mirror throws back the sun's rays into one point or focus, where paper or gunpowder may be set on fire. Mirrors are sometimes made of a cylindrical concave form; and as one of them is placed either upright or on its side, the image of the picture is distorted into a very long or a very broad image. Reflecting surfaces may be made of various shapes, and if a regular figure be placed before an irregular reflector, the image will be deformed; but if an object, as a picture, be painted deformed, according to certain rules, the image will appear regular. Such figures and reflectors are sold by opticians, and they serve to astonish those who are ignorant of these subjects.

Small convex reflectors are made for the use of travellers, who, when fatigued by stretching the eye to Alps towering on Alps, can by their mirror, bring these sublime objects into a narrow compass, and gratify the sight by pictures which the art of man in vain attempts to imitate.

QUESTIONS.-1. What are the three kinds of mirrors? 2. How do convex mirrors make objects appear?-concave? 3. What are some of the experiments that may be performed with them? 4. How do cylindrical concave mirrors make an image of a picture appear? [NOTE. A mirror is sometimes called a Spec'ulum, pl. Spec'ula.] 5. Describe fig. 27.

Sem'icircle, a half round, part of a circle divided by the diameter.
Junct'ure, the line at which two things are joined together.
Prism, a solid piece of glass with three flat sides, and two equal
and parallel triangular ends.

SIR ISAAC NEWTON, to whom we are indebted for the most important discoveries respecting light and colours, was the first who divided a white ray of light, and found it to consist of an assemblage of coloured rays. This separation may be observed in the well known experiment of the prism. A ray being let into a darkened room, through a small round aperture in the shutter, and falling on a triangular glass prism, is, by the refraction of the prism, considerably dilated, and it will exhibit, on a skreen or on the opposite wall, an oblong image called a spectrum, variously coloured; the extremities of which are bounded by semicircles, and the sides are rectilinear. The colours are commonly divided into seven, which, however, have various shades gradually intermixing at their juncture. The following lines from Blackmore represent their order, beginning at the side of the refracting angle of the prism.

Of parent colours, first the flaming red

Sprung vivid forth; the tawny orange, next;
And next, delicious yellow; by whose side
Fell the kind beams of all-refreshing green;
Then the pure blue, that swells autumnal skies,
Ethereal played; and then, of sadder hue,
Emerged the deepened indigo, as when
The heavy skirted evening droops with frost,
While the last gleamings of refracted light
Died in the fainting violet away.

The union of these colours, in the proportions in which they appear in the spectrum, produce in us the idea of whiteness. If you paint a card in compartments with these seven colours, and whirl it rapidly on a pin, it will appear white. But a more decided proof of the composition of a white ray is afforded by uniting these coloured rays, and forming with them a ray of white light. This can be done