move at the same distance from each other. Rays which continually approach each other are said to converge; and when they continually recede from each other, they are said to diverge. The point at which converging rays meet is called the focus.

4. Any pellucid or transparent body, as air, water, and glass, which admits the

2. Light is an emanation from the sun and other luminous bodies; and is that sub-free passage of light, is called a medium. stance which renders opaque bodies visible to the eye. It diverges in a direct line, unless interrupted by some obstacle, and its motion has been estimated at two hundred thousand miles in a second..

3. A ray of light is the motion of a single particle; and a parcel of rays passing from a single point is called a pencil of rays. Parallel rays are such as always

When rays, after having passed through one medium, are bent out of their original course by entering another of different density, they are said to be refracted; and when they strike against a surface, and are sent back from it, they are said to be reflected.

5. A lens is glass ground in such a form as to collect or disperse the rays of light

object towards the lens, passes through it, and concentrates, within the room, in a focal point; at which, if a sheet of paper, or any other plane of a similar colour, be placed, the image of the object will be seen upon it.

which pass through it. These are of dif- || All the light reflected from the latter ferent shapes; and they have, therefore, received different appellations. A planoconvex lens has one side flat, and the other convex: a plano-concave lens is flat on one side, and concave on the other: a double convex lens is convex on both sides: a double concave lens is concave on both sides: a meniscus is convex on one side, and concave on the other. By the following cut, the lenses are exhibited in the order in which they have been mentioned.

HOD

6. An incident ray is that which comes from any luminous body to a reflecting surface; and that which is sent back from a reflecting surface is called a reflected ray. The angle of incidence is the angle which is formed by the incident ray with a perpendicular to the reflecting surface; and the angle of reflection is the angle formed by the same perpendicular and the reflected ray.

7. When the light proceeding from every point of an object placed before a lens is collected in corresponding points behind it, a perfect image of the object is there produced. The following cut is given by way of illustration.

9. This phenomenon is called the camera obscura, or dark chamber, because it is necessary to darken the room to exhibit it. The image at the focal point within the room is in an inverted position. The reason why it is thrown in this manner will be readily understood by observing the direction of the reflected rays, as they pass from the object through the lens. In the camera obscura, it is customary to place a small mirror immediately behind the lens, so as to throw all the light which enters, downwards upon a whitened table, where the picture may be conveniently contemplated,

10. From the preceding explanation of the camera obscura, the theory of vision may be readily comprehended, since the eye itself is a perfect instrument of this kind. A careful examination of the following representation of the eye will render the similarity obvious. The eye is supposed to be cut through the middle, from above downwards.

8. The lens, a, may be supposed to be placed in the hole of a window-shutter of a darkened room, and the arrow at the right to be standing at some distance without.

aa, the sclerotica; b b, the choroides; cc, the retina; dd, the cornea; e, the pupil; ff, the iris; g, the aqueous humour; h, the crystalline humour; ii, the vitreous humour.

11. The sclerotica is a membranous coat, to which the muscles are attached which move the eye. The cornea is united to the sclerotica around the circular open

ing of the latter, and is that convex part 15. The visual point, or the distance at of the eye, which projects in advance of which small objects can be distinctly seen, the rest of the organ. The space between varies in different individuals. As an avethis and the crystalline lens is occupied by || rage, it may be assumed at eight or nine the aqueous humour and the iris. The iris inches from the eye. In some persons, it is united to the choroides, and it possesses is much nearer, and in others, considerably the power of expanding and contracting, more distant. The extreme, in the former to admit a greater or less number of rays.|| case, constitutes myopy, or short-sighted12. The crystalline lens is a small body ness; and, in the latter case, presbyopy, or of a crystalline appearance and lenticular long-sightedness. shape, whence its name. It is situated between the aqueous and vitreous humours, and consists of a membranous sack filled with a humour of a crystalline appearance. The vitreous humour has been thus denominated on account of its resemblance to glass in a state of fusion. The retina is a membrane which lines the whole cavity of the eye, and is formed chiefly, if not entirely, by the expansion of the optic nerve.

16. Myopy is chiefly caused by too great a convexity of the cornea and the crystalline lens, which causes the rays to converge to a focus before they reach the retina. Objects are therefore indistinctly seen by myoptic persons, unless held very near the eye to throw the focus farther back. This defect may be palliated by the use of concave glasses, which render the rays, proceeding from objects, more divergent.

17. Presbyopy is principally caused by too little convexity of the cornea and crystalline lens, which throws the focal point

13. The rays of light which proceed from objects pass through the cornea, aqueous humour, crystalline lens, and vitreous humour, and fall upon the retina in a focal point, || of rays reflected from near objects, beyond to which it is brought chiefly by the influ- || the retina. This defect is experienced by ence of the cornea and the crystalline lens. most people, to a greater or less degree, The image, in an inverted position, is after they have advanced beyond the fortipainted or thrown on the cornea, which eth year, and occasionally even by youth. perceives its presence, and conveys an im- A remedy, or, at least, a palliation, is found pression of it to the brain, by means of the in the use of convex glasses, which render optic nerve. the rays more convergent, and enable the 14. Optical instruments. The art of eye to refract them to a focus farther forconstructing optical instruments is found-ward, at the proper point. ed upon the anatómical structure, and physiological action of the eye, and on the laws of light. They are designed to increase the powers of the eye, or to remedy some defect in its structure. In the cursory view which we may give of a few of the many optical instruments which have been invented, we will begin with the spectacles; since they are best known, and withal the most simple.

18. The opticians have their spectacles numbered to suit different periods of life; but, as the short-sighted and long-sighted conditions exist in a thousand different degrees, each person should select for himself such as will enable him to read without effort at the usual distance.

19. The great obstacle to viewing small objects at the usual distance, arises from too great a divergence of the light reflect

general construction is the same with that of the solar microscope; but in the application, the light of a lamp is employed instead of that from the sun.

ed from them, which causes the rays to reach the retina before they have converged to a focus. This defect is remedied by convex lenses, which bring the visual point nearer to the eye, and consequently 24. Telescopes are employed for viewcause the rays to concentrate in a large ing objects which from their distances apfocus upon the retina. The most powerful pear small, or are invisible to the naked microscopic lenses are small globules of eye. They are of two kinds—refracting glass, which permit the eye to be brought || and reflecting. The former kind is a comvery near to the object. pound of the camera obscura and the single microscope. It consists of a tube, having at the further end a double convex lens, which concentrates the rays at a focal point within, where the image is viewed through a microscopic lens, placed at the other end.

20. Microscopes are either single or double. In the former case, but one lens is used, and through this the object is viewed directly; but, in the latter case, two or more glasses are employed, through one of which a magnified image is thrown upon a reflecting surface, and this is viewed through the other glass, or glasses, as the real object is seen through a single microscope.

21. The solar microscope, on account of its great magnifying powers, is the most wonderful instrument of this kind. The principles of its construction are the same with those of the camera obscura. The difference consists chiefly in the minor circumstance of placing the object very near the lens, by which a magnified image is thrown at the focal point within the

room.

25. In the construction of reflecting telescopes, concave mirrors or specula are combined with a double convex lens. A large mirror of this kind is so placed in the tube that it receives the rays of light from objects, and reflects them upon another of a smaller size. From this they are thrown to a focal point, where the image is viewed through a double convex lens. The specula are made of speculum metal, which is a composition of certain proportions of copper and tin.

26. Many optical appearances are of so frequent recurrence that they could not have escaped the earliest observers; nevertheless, ages appear to have elapsed before any progress was made towards an explanation of them. Empedocles, a Greek philosopher, born at Agrigentum in Sicily, 460 years before Christ, is the first person on record who attempted to write system

22. In the case of the camera obscura, the objects are at a far greater distance from the glass on the outside, than the images, at the focal point, on the inside. The comparatively great distance of the object, in this case, causes the image to be proportionably smaller. In the solar microscope, a small mirror is used to re-atically on light. ceive the rays, and to reflect them directly upon the object.

23. The magic lantern is an instrument used for magnifying paintings on glass, and for throwing their images upon a white surface in a darkened room. Its

27. The subject was successively treated by several other philosophers; but the ancients never attained to a high degree of information upon it. We have reason to believe, however, that convex lenses were, in some cases, used as magnifiers,

and as burning-glasses; although the theory of their refractive power was not understood.

28. The magnifying power of glasses, and some other optical phenomena, were largely treated by Alhazen, an Arabian philosopher, who flourished about the year 1100 of our era: and, in 1270, Vitellio, a Polander, published a treatise on optics, containing all that was valuable in Alhazen's work, digested in a better manner, and with more lucid explanations of various phenomena.

those by Maurolicus, although neither of these philosophers had any knowledge of what the other had done. The importance of Porta's discoveries will be evident, when it is observed, that, before his time, vision was supposed to be dependent on what were termed visual rays, proceeding from the eye.

32. The telescope was invented towards the latter end of the sixteenth century. Of this, as of many other valuable inventions, accident furnished the first hint. It is said that the children of Zacharias Jan29. Roger Bacon, an English monk, who sen, a spectacle-maker of Middleburg, in was born in 1214, and who lived to the Holland, while playing with spectacleage of seventy-eight, described very accu- glasses in their father's shop, perceived rately the effects of convex and concave that, when the glasses were held at a cerlenses, and demonstrated by actual experi- tain distance from each other, the dial of ment, that a small segment of a glass || the clock appeared greatly magnified, but globe would greatly assist the sight of old in an inverted position. persons. Concerning the actual inventor of spectacles, however, we have no certain information; we only know, that these useful instruments were generally known || in Europe about the beginning of the four-tube completed the invention of the simteenth century.

30. In the year 1575, Maurolicus, a teacher of mathematics at Messina, published a treatise on optics, in which he demonstrated that the crystalline humour of the eye is a lens, which collects the rays of light from external objects, and throws them upon the retina. Having arrived at a knowledge of these facts, he was enabled to assign the reasons why some people were short-sighted, and others longsighted.

31. John Baptista Porta, of Naples, was contemporary with Maurolicus. He invented the camera obscura, and his experiments with this instrument convinced him that light was a substance, and that its reception into the eye produced vision. These discoveries corresponded very nearly with

33. This incident suggested to their father the idea of adjusting two of these glasses on a board, so as to move them at pleasure. Two such glasses inclosed in a

plest kind of the refracting telescope. Galileo greatly improved the telescope, and constructed one that magnified thirtythree times, and with this he made the astronomical discoveries which have immortalized his name.

34. John Kepler, a great mathematician and astronomer, who was born at Weir, in Wurtemburg, in the year 1571, paid great attention to the phenomena of light and vision. He was the first who demonstrated that the degree of refraction suffered by light in passing through lenses, corresponds with the diameter of the circle of which the concavity or convexity is the portion of an arch. He very successfully pursued the discoveries of Maurolicus and Porta, and asserted, that the images of external objects were formed upon the