REFLECTION OF LIGHT.

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as an elastic ball which is struck against a wall. A ray of light striking perpendicularly upon a plane mirror, is reflected back in the same direction; but those rays which strike it obliquely, are reflected back in an opposite direction, but with the same obliquity; the angle of reflection, therefore, is exactly equal to the angle of incidence. If you stand directly before a looking-glass, you see your image reflected back to you. If you stand a little to the side, you cannot see yourself; but a person who stands just as far on the other side of it, can see your image in the glass, and you can see his. If you place a candle a little to one side, you must go as far on the other to see its image in the glass. This is the same rule which takes place in the collision of elastic bodies against any surface. If you strike an ivory ball or common marble perpendicularly against the wainscot, it returns to you; but if you make it strike sideways, it goes off at the same angle with which it came to the wainscot. So it is with rays of light; the incident ray, or the ray which falls upon a surface, makes an angle with a perpendicular line, drawn from the point where it strikes, equal to that which the reflected ray makes with it.

With respect to a looking-glass, it is the silvering on the glass which causes the reflection, otherwise the rays would pass through it without being stopped, and if they were not stopped they could not be reflected. No glass, however, is so transparent but it reflects some rays: if you put your hand near a window, you clearly see its image on the other side, and the nearer the hand is to the glass, the more evi dent is the image. Whatever suffers the rays of light to pass through it is called a medium, and the more transpa rent the body, the more perfect is the medium. But rays of light do not pass through a transparent medium, (unless they fall perpendicularly upon it) in precisely the same direction in which they were moving before they entered it. They are bent out of their former course, and this is called refraction. When they pass out of a rarer into a denser medium, as from air into water or glass, they are always res fracted towards a perpendicular to the surface, and the res fraction is, more or less, in proportion as the rays fall, more or less, obliquely on the refracting surface. But when they pass from a denser into a rarer medium, as from glass of water into air, they move in a direction farther from the

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REFRACTION OF LIGHT.

perpendicular. If you put a piece of money into an empty basin, and stand at such a distance that it may not be visible; then let another person pour water into the basin, and the money will be seen; for the rays of light, in passing from a denser into a rarer medium, are bent from the perpendicular, and thus are directed to your eye. The following, therefore, may be established as a sort of axiom in optics we see every thing in the direction of that line in which the rays approach us last. If you place a candle before a looking-glass, and stand before it, the image of the candle appears behind it; but if another looking-glass be so placed as to receive the reflected rays of the candle, and you stand before this second glass, the candle will appear behind that; because the mind imagines every object to be in the direction from which the rays come to the eye last. Hence, when the rays of light coming from the celestial bodies, arrive at our atmosphere, they are bent downward; and those bodies appear, when in the horizon, higher than they are. The effect of this refraction is about six minutes of time, but the higher they rise, the less are the rays refracted; and when they are in the zenith, they suffer no refraction. The sun is visible about three minutes before he rises, and about the same time after he sets; making in the course of a year about a day and a half. Twilight is occasioned partly by refraction, but chiefly by reflection of the sun's rays by the atmosphere, and it lasts till the sun is eighteen degrees below the horizon. Were there no atmosphere to reflect and refract the sun's rays, only that part of the heavens would be luminous in which the sun is placed; and if we could live without air, and should turn our backs to the sun, the whole heavens would appear as dark as in the night. In this case also, a sudden transition from the brightest sunshine to dark night would immediately take place upon the setting of the sun.

QUESTIONS.-1. What is said of optics? 2. In what manner is light projected from luminous bodies? 3. What is still a disputed point, and what is said of it? 4. How are rays of light reflected? 5. How is it shown that the angle of reflection is equal to the angle of incidence? 6. What is meant by the refraction of rays of light? 7. How are they refracted in passing from a rarer into a denser medium? 8. From a denser into a rarer? 9. What is the example for illustration? 10. What may be established as a sort of axiom in optics? 11. Give the illustration. 12. What is the effect of rays of light, coming from

,

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 convex 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 pow erful in their refractions, than those which have been de scribed. 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 mēnis'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.

Optician, 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