gent upon the crystalline humour, and consequently include a greater angle, and thus the object is magnified. In objects placed at such distances as we are used to, we know, by experience, how much an increase of distance will diminish their apparent magnitude, and we instantly suppose them of the size they would appear if they were less remote; but this can only be done, where we are well acquainted with the real magnitude of the object; in all other cases we judge of magnitudes by the angle under which the object appears at the known, or supposed distance; that is, we infer the real magnitude from the apparent magnitude in comparison with the distance of the object. Sight, therefore, does not represent extension such as it is in itself; it often deceives us both in regard to the size and the distance of objects, and we should be led into continual errors if experience did not set us right. This is rendered strikingly manifest from the case of a young man who was blind from his infancy, and who recovered his sight at the age of fourteen, by the operation of couching. At first he had no idea either of the size or distance of objects, but imagined that every thing he saw touched his eyes; and it was not till after having repeatedly felt them and walked from one object to another, that he acquired an idea of their respective dimensions, their relative situations, and their distances.

QUESTIONS.-1. What is the external covering of the eye called?Describe it. 2. Describe the cornea. 3. The choroid. 4. The pupil. 5. The iris. 6. What is said in order to illustrate the contraction and dilatation of the iris? 7. Of what use is the black liquor in the choroid? 8. Describe the three humours of the eye. 9. Of what does the retina consist, and what is its use? dered distinct? 10. How is the image on the retina ren11. How does it appear that the image on the retina will be inverted? 12. Having two eyes, why do we not see objects double? 13. Why does a distant object appear smaller than one that is near? 14. How do we judge of the real magnitudes of objects? 15. What case is related to show that experience is necessary to correct the errors of sight? 16. Look at fig. 28. and describe the eye. [NOTE. Let the instructer explain to his pupils how objects of equal magnitudes appear under a greater angle when near, than when at a distance.]

SPECTACLES.

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LESSON 37.

Optical Instruments.

Land'scape, the prospect of a country,-also a picture representing an extent of space with the various objects on it.

Glob'ule, a small particle of matter of a globular or spherical figure.

As the sight is the most noble and extensive of all our senses; as we make the most frequent use of our eyes in all the actions and concerns of life; that instrument which relieves the eyes when decayed, and supplies their defects, must be estimated as one of the greatest of advantages. Sight may be defective in various ways. Some eyes are too flat, others are too convex or round; in some, the humours lose a part of their transparency, and on that account, much of the light that enters the eye is stopped and lost in the passage, and every object appears dim. Spectacles are intended to collect the light and to bring it to a proper degree of convergency. The honour of their invention was claimed by Salvinus Armatus, a nobleman of Florence, who died in 1317, and the fact was inscribed on his tomb. When the eye is too flat, the rays proceeding from objects do not converge to a focus so soon as they reach the retina; in this case a convex glass is necessary, for it has the property of converging the rays, and of course, when suited to the eye, of bringing them to a focus, and forming an image on the retina. When the eye is too convex, the rays of light are converged to a focus before they reach the retina; to remedy this, a concave glass is used, which causes the rays to diverge, and prevents their coming to a focus too soon. Shortsighted persons bring objects close to their eyes; it has a similar effect to that produced by concave glasses; for the nearer an object is brought to the eye, the greater is the angle under which it is seen, that is, the extreme rays, and of course all the others, are made more divergent. But persons whose eyes are too flat, when examining an object, hold it at a distance, for the farther an object is held from their eyes, the less is the divergency of its rays, that is, the smaller is the angle under which it is seen the focal distance is increased, and an image is properly formed on the retina. In considering vision as achieved by the means of an image formed

at the bottom of the eye, we can never reflect, without wonder, upon the smallness, yet correctness of the picture, the subtilty of the touch, the fineness of the lines. A landscape of five or six square leagues is brought into a space of half an inch diameter; yet the multitude of objects which it contains are all preserved; are all discriminated in their magnitudes, positions, figures, colours.

Microscopes are instruments for viewing small objects; and they apparently magnify them, because they enable us to see them nearer than with the naked eye, without affecting the distinctness of vision. The distance from the naked eye, at which most persons are supposed to see small objects best, is about seven inches; but by the help of convex glasses, we are enabled to view things clearly at a much shorter distance than this; for the nature of a convex lens is, to render an object distinctly visible to the eye at the distance of its focus. With a knowledge of this fact, we may easily determine the magnifying powers of glasses employed in Single Microscopes, which are small double convex lenses, having the object placed in the focus, and the eye at the same distance on the other side. If rays of light from an object are converged to a point at the distance of one inch from the centre of the glass, or, in other words, if the focal distance of the lens is one inch, an object may be seen through that lens at one inch distance from the eye, and it will appear, in its diameter,—since the natural sight is seven inches,- -seven times larger than to the naked eye. But as the object is magnified every way equally, in length as well as breadth, we must square this diameter, to know really how much the object appears enlarged; and we shall thus find that its surface is magnified forty-nine times. If we suppose the focus of a convex lens to be at one-tenth of an inch distant from its centre, in seven inches there are seventy such tenths of an inch; and an object therefore may be seen through this lens seventy times nearer than it can, distinctly, by the naked eye. It will consequently appear seventy times longer and seventy times broader than it does to common sight; and as seventy multiplied by seventy makes four thousand nine hundred; so many times it really appears magnified. Those lenses, therefore, which have the shortest focus, will magnify the object most. Single microscopes of the greatest power may be made with a very small

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globule of glass, fixed in a thin plate of metal, so that the middle of it may be directly over the centre of an extremely small hole made in the plate.

The compound microscope consists of at least two lenses, by one of which an image is formed within the tube of the microscope; and this image is viewed through the eye-glass, instead of the object itself. The solar microscope is a kind of camera obscura, which, in a darkened chamber, throws the image on a wall or skreen. It consists of two lenses fixed opposite to a hole in a board or window-shutter. There is also a plane reflector or mirror placed without, which may be so regulated as to throw the sun's rays upon the outer lens. A magic lantern is constructed on the same principles. The light is supplied by a lamp instead of the sun, and it is used for magnifying paintings on glass, and throwing their images upon a white skreen in a darkened chamber.

QUESTIONS.-1. In what ways may sight be defective? 2. For what are spectacles intended? 3. How do they assist eyes that are too flat? 4. Too convex or round? 5. Why do some persons bring objects close to their eyes, and others hold them at a distance? 6. What are microscopes? 7. Single microscopes? 8. How is their magnifying power calculated? 9. Describe the compound microscope. 10. Solar microscope. 11. Magic lantern. 12. Look on fig. 35. and describe the single microscope. 13. On fig. 34. and describe the compound microscope.

LESSON 38.

Microscopic Discoveries.

Miniature, (pronounced mine-ture,) representation in a small compass. Filament, a slender thread.

Ped'icle, a footstalk. Animal'cule, a small animal.

Con'ical, consisting of a circular base or bottom and ending in a point.

Tissue, (pron. tish'ū,) a substance interwoven with threads, pr variegated.

THE microscope has opened to us a new world of insects and vegetables; it has taught us that objects, invisible to the naked eye, exist, having figure, extension, and different parts; some examples of which we shall produce, that we may have more reasons for admiring and praising the wisdom and power of God. A grain of sand when examined by the eye appears round, but with the help of a glass we

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MICROSCOPIC DISCOVERIES:

observe that each grain differs from the other, both in size and figure; some of them are perfectly round, others square, some conical, and the greater part of an irregular form. By microscopes which magnify objects millions of times, we can discover in the grains of sand a new animal world; for within their cavity dwell various insects. In the vegetable kingdom we are presented with a thick forest of trees and plants, bearing leaves, branches, flowers, and fruits. Mouldiness, when looked at by the naked eye, seems nothing but an irregular tissue of filaments; but the magnifying glass shows it to be a forest of small plants, which derive their nourishment from the moist substance which serves them

as a base. The stems of these plants may be plainly distinguished, and sometimes their buds, some shut and some open. They have much similarity to mushrooms, which, it is well known, are the growth of a single night; but those in miniature, of which we are speaking, seem to come to perfection in a much less space of time; hence we account for the extraordinary progress which mouldiness makes in a few hours. A sort of dust, which covers some stones, has been found to consist of small mushrooms, raised on pedicles, the heads of which, round the middle, were turned up at the edges. Above their covering a multitude of small grains appear, shaped like cherries somewhat flattened; and among them several small red insects, which probably feed upon them. A small drop of the green surface of water, that has stood: for some time, has been found to be altogether composed of animalcules of several shapes and magnitudes. The most remarkable were those that gave the water the green colour; they were oval creatures; they could contract and dilate themselves, tumble over many times together, and then shoot away like fishes.

If you slightly bruise some corns of pepper, and infuse them in water for a few days, and then expose a drop of it to the microscope, a number of animalcules will be visible, in continual motion, going backwards and forwards in all directions, turning aside when they meet each other, or when their passage is stopped by some obstacle. In other infusions, as in that of new hay, differently shaped animalcules will be found. When the drop in which they swim, and which to them is like a pond, becomes diminished by evaporation, they gradually retire towards the middle, where they