Part 23

_Mrs. B._ It is a double microscope, (fig. 6.) in which you see, not the object A B, but a magnified image of it, _a b_. In this microscope, two lenses are employed; the one, L M, for the purpose of magnifying the object, is called the object-glass, the other, N O, acts on the principle of the single microscope, and is called the eye-glass.

There is another kind of microscope, called the solar microscope, which is the most wonderful from its great magnifying power: in this we also view an image formed by a lens, not the object itself. As the sun shines, I can show you the effect of this microscope; but for this purpose, we must close the shutters, and admit only a small portion of light, through the hole in the window-shutter, which we used for the camera obscura. We shall now place the object A B, (plate 23, fig. 1.) which is a small insect, before the lens C D, and nearly at its focus: the image E F, will then be represented on the opposite wall, in the same manner, as the landscape was in the camera obscura; with this difference, that it will be magnified, instead of being diminished. I shall leave you to account for this, by examining the figure.

_Emily._ I see it at once. The image E F is magnified, because it is farther from the lens, than the object A B; while the representation of the landscape was diminished, because it was nearer the lens, than the landscape was. A lens, then, answers the purpose equally well, either for magnifying or diminishing objects?

_Mrs. B._ Yes: if you wish to magnify the image, you place the object near the focus of the lens; if you wish to produce a diminished image, you place the object at a distance from the lens, in order that the image may be formed in, or near the focus.

_Caroline._ The magnifying power of this microscope is prodigious: but the indistinctness of the image, for want of light, is a great imperfection. Would it not be clearer, if the opening in the shutter were enlarged, so as to admit more light?

_Mrs. B._ If the whole of the light admitted, does not fall upon the object, the effect will only be to make the room lighter, and the image consequently less distinct.

_Emily._ But could you not by means of another lens, bring a large pencil of rays to a focus on the object, and thus concentrate upon it the whole of the light admitted?

_Mrs. B._ Very well. We shall enlarge the opening, and place the lens X Y (fig. 2.) in it, to converge the rays to a focus on the object A B. There is but one thing more wanting to complete the solar microscope, which I shall leave to Caroline's sagacity to discover.

_Caroline._ Our microscope has a small mirror attached to it, upon a moveable joint, which can be so adjusted as to receive the sun's rays, and reflect them upon the object: if a similar mirror were placed to reflect light upon the lens, would it not be a means of illuminating the object more perfectly?

_Mrs. B._ You are quite right. P Q (fig. 2.) is a small mirror, placed on the outside of the window-shutter, which receives the incident rays S S, and reflects them on the lens X Y. Now that we have completed the apparatus, let us examine the mites on this piece of cheese, which I place near the focus of the lens.

_Caroline._ Oh, how much more distinct the image now is, and how wonderfully magnified! The mites on the cheese look like a drove of pigs scrambling over rocks.

_Emily._ I never saw any thing so curious. Now, an immense piece of cheese has fallen: one might imagine it an earthquake: some of the poor mites must have been crushed; how fast they run--they absolutely seem to gallop.

But this microscope can be used only for transparent objects; as the light must pass through them, to form the image on the wall?

_Mrs. B._ Very minute objects, such as are viewed in a microscope, are generally transparent, but when opaque objects are to be exhibited, a mirror M N (fig. 3.) is used to reflect the light on the side of the object next the wall: the image is then formed by light reflected from the object, instead of being transmitted through it.

_Emily._ Pray, is not a magic lanthorn constructed on the same principles?

_Mrs. B._ Yes, with this difference; the objects to be magnified, are painted upon pieces of glass, and the light is supplied by a lamp, instead of the sun.

The microscope is an excellent invention to enable us to see and distinguish objects, which are too small to be visible to the naked eye. But there are objects, which, though not really small, appear so to us, from their distance; to these, we cannot apply the same remedy; for when a house is so far distant, as to be seen under the same angle as a mite which is close to us, the effect produced on the retina is the same: the angle it subtends is not large enough for it to form a distinct image on the retina.

_Emily._ Since it is impossible, in this case, to make the object approach the eye, cannot we by means of a lens bring an image of it, nearer to us?

_Mrs. B._ Yes; but then the object being very distant from the focus of the lens, the image would be too small to be visible to the naked eye.

_Emily._ Then, why not look at the image through another lens, which will act as a microscope, enable us to bring the image close to the eye, and thus render it visible?

_Mrs. B._ Very well, Emily; I congratulate you on having invented a telescope. In figure 4, the lens C D, forms an image E F, of the object A B; and the lens X Y, serves the purpose of magnifying that image; and this is all that is required in a common refracting telescope.

_Emily._ But in fig. 4, the image is not inverted on the retina, as objects usually are: it should therefore appear to us inverted; and that is not the case in the telescopes I have looked through.

_Mrs. B._ When it is necessary to represent the image erect, two other lenses are required; by which means a second image is formed, the reverse of the first, and consequently upright. These additional glasses are used to view terrestrial objects; for no inconvenience arises from seeing the celestial bodies inverted.

_Emily._ The difference between a microscope and a telescope, seems to be this:--a microscope produces a magnified image, because the object is nearest the lens; and a telescope produces a diminished image, because the object is furthest from the lens.

_Mrs. B._ Your observation applies only to the lens C D, or object-glass, which serves to bring an image of the object nearer the eye; for the lens X Y, or eye-glass, is, in fact, a microscope, as its purpose is to magnify the image.

When a very great magnifying power is required, telescopes are constructed with concave mirrors, instead of lenses. These are called reflecting telescopes, because the image is reflected by metallic mirrors. Concave mirrors, you know, produce by reflection, an effect similar to that of convex lenses, by refraction. In reflecting telescopes, therefore, mirrors are used in order to bring the image nearer the eye; and a lens, or eye-glass, the same as in the refracting telescope, to magnify the image.

The advantage of the reflecting telescope is, that mirrors whose focus is six feet, will magnify as much as lenses of a hundred feet: an instrument of this kind may, therefore, possess a high magnifying power, and yet be so short, as to be readily managed.

_Caroline._ But I thought it was the eye-glass only which magnified the image; and that the other lens, served to bring a diminished image nearer to the eye.

_Mrs. B._ The image is diminished in comparison with the object, it is true; but it is magnified, if you compare it to the dimensions of which it would appear without the intervention of any optical instrument; and this magnifying power is greater in reflecting, than in refracting telescopes.

We must now bring our observations to a conclusion, for I have communicated to you the whole of my very limited stock of knowledge of Natural Philosophy. If it enable you to make further progress in that science, my wishes will be satisfied; but remember, in order that the study of nature may be productive of happiness, it must lead to an entire confidence in the wisdom and goodness of its bounteous Author.

Questions

1. (Pg. 195) What is the form of the body of the eye? fig. 1, plate 21.

2. (Pg. 195) What is its external coat called?

3. (Pg. 195) What is the transparent part of this coat denominated?

4. (Pg. 195) What is the second coat named?

5. (Pg. 195) What opening is there in this?

6. (Pg. 195) What is the coloured part which surrounds the pupil?

7. (Pg. 195) The pupils dilate and contract, what purpose does this answer?

8. (Pg. 196) How could you observe the dilatation and contraction of the pupils?

9. (Pg. 196) What purpose is the choroid said to answer?

10. (Pg. 196) In what animals is the change in the iris greatest?

11. (Pg. 196) What are the three humours denominated, and how are they situated?

12. (Pg. 197) What is the part represented at _i i_, and of what does it consist?

13. (Pg. 197) What are the respective uses of the humours, and of the retina?

14. (Pg. 197) Why is it necessary the rays should be refracted?

15. (Pg. 197) How is this illustrated by fig. 3 and 4, plate 21?

16. (Pg. 198) What causes a person to be short-sighted? fig. 5, plate 21.

17. (Pg. 198) Why does placing an object near the eye, enable such, to see distinctly? fig. 6.

18. (Pg. 199) A concave lens remedies this defect; how? fig. 1, plate 22.

19. (Pg. 199) What is the remedy, when a person is long-sighted? fig. 2.

20. (Pg. 199) Why does holding an object far from the eye, help such persons? fig. 3.

21. (Pg. 200) How is the eye said to adapt itself to distant, and to near objects?

22. (Pg. 200) Why are objects rendered indistinct, when placed very near to the eye? fig. 4, plate 22.

23. (Pg. 200) What is the single microscope, fig. 5, and how does it magnify objects?

24. (Pg. 201) How may objects be magnified without the aid of a lens?

25. (Pg. 201) Why can an object, very near to the eye, be distinctly seen, when viewed through a small hole?

26. (Pg. 201) Describe the double microscope, as represented in fig. 6, plate 22.

27. (Pg. 202) How does the solar microscope, (fig. 1 plate 23.) operate?

28. (Pg. 202) Why may minute objects be greatly magnified by this instrument?

29. (Pg. 202) In its more perfect form it has other appendages, as seen in fig. 2, what are they? and what their uses?

30. (Pg. 203) What is added when opaque objects are to be viewed? fig. 3.

31. (Pg. 203) In what does the magic lanthorn differ from the solar microscope?

32. (Pg. 203) What are the use and structure of the telescope, as shown in fig. 4?

33. (Pg. 204) When terrestrial objects are to be viewed, why are two additional lenses employed?

34. (Pg. 204) What part of the telescope performs the part of a microscope?

35. (Pg. 204) In what does the reflecting, differ from the refracting telescope?

36. (Pg. 204) What advantages, do reflecting, possess over refracting telescopes?

GLOSSARY.

ACCELERATED MOTION. Motion is said to be accelerated, when the velocity is continually increasing.

ACCIDENTAL PROPERTIES. Those properties of bodies which are liable to change, as colour, form, &c.

ACUTE.--See ANGLE.

AIR. An elastic fluid. The atmosphere which surrounds the earth, is generally understood by this term, but there are many kinds of air. The term is synonymous with _Gas_.

AIR PUMP. An instrument by which vessels may be exhausted of air.

ALTITUDE. The height in degrees of the sun, or any heavenly body, above the horizon.

ANGLE. The space contained between two lines inclined to each other, and which meet in a point. Angles are measured in degrees, upon a segment of a circle described by placing one leg of a pair of compasses on the angular point, and with the other, describing the segment between the two lines. If the segment be exactly 1-4th of a circle, it is called a _right_ angle, and contains 90 deg. If more than 1-4th of a circle, it is an _obtuse_ angle. If less, an _acute_ angle. See plate 2.

ANGLE OF INCIDENCE, is the space contained between a ray which falls obliquely upon a body, and a line perpendicular to the surface of the body, at the point where the ray falls.

ANGLE OF REFLECTION. The space contained between a reflected ray, and a line perpendicular to the reflecting point.

ANGLE OF VISION, or visual angle. The space contained between lines drawn from the extreme parts of any object, and meeting in the eye.

ANTARCTIC CIRCLE. A circle extending round the south pole, at the distance of 23 1-2 degrees from it. The same as the south frigid zone.

APHELION. That part of the orbit of a planet, in which its distance from the sun is the greatest.

AREA. The surface enclosed between the lines which form the boundary of any figure, whether regular or irregular.

ARIES. See SIGN.

ASTEROIDS. The name given to the four small planets, Ceres, Juno, Pallas, and Vesta.

ASTRONOMY. The science which treats of the motion and other phenomena of the sun, the planets, the stars, and the other heavenly bodies.

ATMOSPHERE. The air which surrounds the earth, extending to an unknown height. Wind is this air in motion.

ATTRACTION. A tendency in bodies to approach each other, and to exist in contact.

ATTRACTION OF COHESION. That attraction which causes matter to remain in masses, preventing them from falling into powder. For this attraction to exist, the particles must be contiguous.

ATTRACTION OF GRAVITATION. By this attraction, masses of matter, placed at a distance, have a tendency to approach each other. Attraction is mutual between the sun and the planets.

AXIS OF THE EARTH, OR OF ANY OF THE PLANETS. An imaginary line passing through their centres, and terminating at their poles; round this their diurnal revolutions are performed.

AXIS OF MOTION. The imaginary line, around which all the parts of a body revolve, when it has a spinning motion.

AXIS OF A LENS, OR MIRROR. A line passing through the centre of a lens, or mirror, in a direction perpendicular to its surface.

BALLOON. Any hollow globe. The term is generally applied to those which are made to ascend in the air.

BAROMETER. Commonly called a weather-glass. It has a glass tube, containing quicksilver, which by rising and falling, indicates any change in the pressure of the atmosphere, and thus frequently warns us of changes in the weather.

BODY. The same as _Matter_. It may exist in the solid, liquid, or aeriform state; and includes every thing with which we become acquainted by the aid of the senses.

BURNING-GLASS, OR MIRROR. A lens, or a mirror, by which the rays of light, and heat, are brought to a focus, so as to set bodies on fire.

CAMERA OBSCURA, a darkened room; or more frequently a box, admitting light by one opening, where a lens is placed; which, bringing the rays of light, from external objects, to a focus, presents a perfect picture of them, in miniature.

CAPILLARY TUBES. Tubes, the bore of which is very small. Glass tubes are usually employed, to show the phenomenon of _capillary attraction_. Fluids in which they are immersed, rise in such tubes above the level of that in the containing vessel.

CENTRE OF A CIRCLE. A point, equally distant from every part of its circumference.

CENTRE OF GRAVITY. That point within a body, to which all its particles tend, and around which they exactly balance each other. A system of bodies, as the planets, may have a common centre of gravity, around which they revolve in their orbits; whilst each, like the earth, has its particular centre of gravity within itself.

CENTRE OF MOTION. That point about which the parts of a revolving body move, which point is, itself, considered as in a state of rest.

CENTRE OF MAGNITUDE. The middle point of any body. Suppose a globe, one side of which is formed of lead, and the other of wood, the centres of magnitude and of gravity, would not be in the same points.

CENTRAL FORCES. Those which either impel a body towards, or from, a centre of motion.

CENTRIFUGAL. That which gives a tendency to fly from a centre.

CENTRIPETAL. That which impels a body, towards a centre.

CIRCLE. A figure; the periphery, or circumference of which, is every where equally distant, from the point, called its centre.

CIRCLE, GREAT. On the globe, or earth, is one that divides it into two equal parts, or hemispheres. The equator, and meridian lines, are great circles.

CIRCLE, LESSER. Those which divide the globe into unequal parts. The tropical, arctic and antarctic circles, and all parallels of latitude, are lesser circles.

CIRCUMFERENCE. The boundary line of any surface, as that which surrounds the centre of a circle; the four sides of a square, &c.

COMETS. Bodies which revolve round the sun, in very long ovals, approaching him very nearly in their perihelion, but in their aphelion, passing to a distance immeasurably great.

COHESION. See ATTRACTION.

COMPRESSIBLE. Capable of being forced into a smaller space.

CONCAVE. Hollowed out; the inner surface of a watch-glass is concave, and may represent the form of a _concave mirror_, or _lens_.

CONVEX. Projecting, or bulging out, as the exterior surface of a watch-glass, which may represent the form of a _convex mirror_, or _lens_.

CONE. A body somewhat resembling a sugar-loaf; that is, having a round base, and sloping at the sides, until it terminates in a point.

CONJUNCTION. When three of the heavenly bodies are in a straight or right line, if you take either of the extreme bodies, the other two are in conjunction with it; because a straight line drawn from it, might pass through the centres of both, and join them together. At the time of new moon, the moon and sun are in conjunction with the earth; and the moon and earth, are in conjunction with the sun.

CONSTELLATION, OR SIGN. A collection of stars. Astronomers have imagined pictures drawn in the heavens, so as to embrace a number of contiguous stars, and have named the group after the animal, or other article supposed to be drawn; an individual star is generally designated by its fancied location; as upon the ear of _Leo_, the Lion, &c.

CONVERGENT RAYS, are those which approach each other, so as eventually to meet in the same point.

CRYSTALS. Bodies of a regular form, having flat surfaces, and well defined angles. Nitre, and other salts, are familiar examples. Many masses of matter, are composed of crystals too minute to be discerned without glasses.

CURVILINEAR, consisting of a line which is not straight, as a portion of a circle, of an oval, or any curved line.

CYLINDER. A body in the form of a roller, having flat circular ends, and being of equal diameter throughout.

DEGREE. If a circle of any size be divided into 360 equal parts, each of these parts is called a degree. One quarter of a circle contains ninety degrees; one twelfth of a circle, thirty degrees. The actual length of a degree, must depend upon the size of the circle. A degree upon the equator, upon a meridian, or any great circle of the earth, is equal to 69-1/2 miles.

Straight lines are sometimes divided into equal parts, called degrees; but these divisions are arbitrary, bearing no relationship to the degrees upon a circle.

DENSITY. Closeness of texture. When two bodies are equal in bulk, that which weighs the most, has the greatest density.

DIAGONAL. A line drawn so as to connect two remote angles of a square, or other four-sided figure.

DILATATION. The act of increasing in size. Bodies in general, dilate when heated, and contract by cooling.

DISCORD. When the vibrations of the air, produced by two musical tones, do not bear a certain ratio to each other, a jarring sound is produced, which is called discord.

DIVERGENT RAYS. Those which proceed from the same point, but are continually receding from each other.

DIVISIBILITY. Capability of being divided, or of having the parts separated from each other. This is called one of the _essential properties_ of matter; because, however minute the particles may be, they must still contain as many halves, quarters, &c. as the largest mass of matter.

ECHO. A sound reflected back, by some substance, so situated as to produce this effect.

ECLIPSE. The interruption of the light of the sun, or of some other heavenly body, by the intervention of an opaque body. The moon passing between the earth and the sun, causes an eclipse of the latter.

ECLIPTIC. A circle in the heavens. The apparent path of the sun, through the twelve signs of the zodiac. This is caused by the actual revolution of the earth, round the sun. It is called the ecliptic, because eclipses always happen in the direction of that line, from the earth.

ELASTICITY. That property of bodies, by which they resume their dimensions and form, when the force which changed them is removed. Air is eminently elastic. Two ivory balls, struck together, become flattened at the point of contact; but immediately resuming their form, they react upon each other.

ELLIPSIS. An oval. This figure differs from a circle, in being unequal in its diameters, and in having two centres, or points, called its _foci_. The orbits of the planets are all elliptical.

EQUATOR. That imaginary line which divides the earth into northern and southern hemispheres, and which is equally distant from each pole.

EQUILIBRIUM. When two articles exactly balance each other, they are in equilibrium. They may, notwithstanding, be very unequal in weight, but they must be so situated, that, if set in motion, their momentums would be equal.

EQUINOX. The two periods of time at which the nights and days are every where of equal length. The _vernal_ equinox is in March, when the sun enters the sign _Aries_; the _autumnal_ equinox in September, when the sun enters _Libra_. At these periods, the sun is vertical at the equator.

EXHALATIONS. All those articles which arise from the earth, and mixing with the atmosphere, form vapour.

EXPANSION. The same as dilatation, which see.

EXTENSION. One of the essential properties of matter; that by which it occupies some space, to the exclusion of all other matter.

FIGURE. All matter must exist in some form, or shape; hence figure is deemed an essential property of matter.

FLUID. A form of matter, in which its particles readily flow, or slide, over each other. Airs, or gases, are called elastic fluids, because they are readily reduced to a smaller bulk by pressure. Liquids, are denominated non-elastic fluids, because they suffer but little diminution of bulk, by any mechanical force.

FOCUS. That point in which converging rays unite.

FORCE. That power which acts upon a body, either tending to create, or to stop motion.

FOUNTAIN. A jet, or stream of water, forced upwards by the weight of other water, by the elasticity of air, or some other mechanical pressure.

FRICTION. The rubbing of bodies together, by which their motion is retarded. Friction may be lessened, but cannot be destroyed.

FRIGID ZONES. The spaces or areas, contained within the arctic and antarctic circles.

FULCRUM. A prop. The point or axis, by which a body is supported, and about which it is susceptible of motion.

GAS. Any kind of air; of these there are several. The atmosphere consists of two kinds, mixed, or combined with each other.

GEOMETRY. That branch of the mathematics, which treats of lines, of surfaces, and of solids; and investigates their properties, and proportions.

GLOBE. A sphere, or ball. It has a point in its centre of magnitude, from which its surface is every where equally distant.

GRAVITY. That species of attraction which appears to be common to matter, existing in its particles, and giving to them, and of course to the masses which they compose, a tendency to approach each other. By gravity a stone falls to the earth, and by it the heavenly bodies tend towards each other.