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 mēnis'cus, being convex on one side and concave on the other, like the glass or crystal of a watch.

the

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 reluc tance 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 cer tain 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 theis 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 Speculum, 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

by letting the coloured rays, which have been separated by a prism, fall upon a lens, which will converge them to a focus, and, being thus re-united, will appear white as they did before refraction.

Prisms are commonly made of solid glass, but those who do not possess one of this kind may easily make a substitute. Take three pieces of plate glass, each four or six inches long, and two or three inches wide; procure a tin frame, the two ends of which are in the exact shape of the three pieces of glass placed in the form of a triangle, with a strip of tin running from each angle of one end to the angles or corners of the other. These strips are bent so as to receive the two edges of the glass plates. The tin forming the ends is turned up so as to receive the plates, and one of the ends is furnished with a little tube to pour in water. When the frame and the glass plates are fastened together, and the crevices stopped, the prism is filled with clear water, and is ready for experiment.

When a spectrum is formed by the light which has passed through a prism upon a skreen, if a small hole be made through the skreen, and the rays of one colour only be permitted to pass through it, then whatever is viewed in that light, will appear of that particular colour. Thus if red light only has passed through the hole, then blood, or grass, or milk, viewed in that light behind the skreen, will appear red; excepting that the blood will appear of a stronger red colour than the grass or milk. If the blue light only has been transmitted through the hole, then the above mentioned substances will appear blue; and the like must be understood of the other colours. This proves that the colours, which seem to proceed from coloured bodies in general, do not belong to those bodies; but they are the com ponent parts of the white light, in which those bodies are viewed, and that certain bodies have the property of absorbing some of those coloured rays of the white light which falls upon them, and of reflecting others. Thus, grass reflects the green rays and absorbs the rest; hence, the green rays coming to our eyes, render the appearance of grass green; thus blood absorbs every other coloured ray excepting the red, and so forth. Black bodies absorb all the seven coloured rays, and white bodies reflect them all. Providence appears to have decorated nature with the enchanting di