called the aqueous humour, from its liquidity and resemblance to water. Beyond this is situated the crystalline humour, so called from its clearness and transparency; it has the form of a lens, and refracts the rays of light in a greater degree of perfection than any that have been constructed by art. The back part of the eye, between the crystalline humour and the retina, is filled by the vitreous humour, which derives its name from its supposed resemblance to glass. The most important part of the eye is the retina; for it is that which receives the impression of the objects of sight, and conveys it to the mind. It consists of an expansion of the optic nerve of the most perfect whiteness it proceeds from the brain, enters the eye and is finally spread over the interior surface of the choroid. The refraction occasioned by the several humours unites the whole of a pencil of rays, proceeding from any one point of an object, to a corresponding point on the retina, and the image is thus rendered distinct and strong. The muscles of the eye are six, and by the excellence of their arrangement it is enabled to move in all directions.

All three of the humours of the eye have some effect in refracting the rays of light, but the crystalline is the most powerful it is a complete double convex lens; and as every point of an object sends out rays in all directions, some rays from each point on the side next the eye will be converged and brought to as many points on the retina, and will form on it a distinct inverted picture of the object, which is seen erect by the habit of the mind. Although an image must be formed on the retina of each of our eyes, yet we do not see objects double; for when an object is seen distinctly with both eyes, the axis of each is directed to it, and the object appears single; but if the axes of both eyes are not directed to the object, it always appears double. If you look at any object, and then by pressing upon the under or upper side of one eye, remove it out of its natural place, you will see two objects, whose distance from each other will vary as the eye is more or less turned from its natural position.

It is well known that an object at a distance appears smaller than when it is near. The reason is, that the nearer any object can be brought to the eye, the larger will be the angle under which it appears; for the rays fall more diver

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? 10. How is the image on the retina rendered distinct? 11. 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 too flat the rays proceeding from objects do not conting in this

1

eyt low saway in verge to a focus so soon as they reach the ice, 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

MICROSCOPIC DISCOVERIES.

83

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 min'e-türe,) representation in a small compass. Fil'ament, 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'ü,) a substance interwoven with threads, or 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