Can all that optics teach, unfold When Science from Creation's face And yet, fair bow, no fabling dreams, When o'er the green undeluged earth And when its yellow lustre smiled Methinks thy jubilee to keep, Nor ever shall the Muse's eye The earth to thee her incense yields, How glorious is thy girdle cast Or mirror'd in the ocean vast, HALO AND PARHELIA. As fresh in yon horizon dark, For faithful to its sacred page, Heaven still rebuilds thy span. CAMPBELL. 77 In serene weather, we often observe a circular light, or luminous ring surrounding the moon; it is called a halo, or crown. Its outline sometimes faintly shows the colours of the rainbow. The moon is in the middle of this ring, and the intermediate space is generally darker than the rest of the sky. When the moon is at the full, and considerably elevated above the horizon, the ring appears most luminous. It is often very large. We are not right in supposing, that this circle really surrounds the moon; the true cause of such an appearance must be looked for in our atmosphere, the vapours of which make a refraction of the rays of light. False moons are sometimes seen near the real moon, and appear as large, but their light is paler. They are generally accompanied by circles, some of which have the same colours as the rainbow, whilst others are white, and others have long luminous tails. All these appearances are produced by refraction. The rays of light falling from the moon upon aqueous and sometimes frozen vapours, are refracted in various ways; the coloured rays are separated, and reaching the eye present a new image of the moon. . Parhelia or mock-suns are far more rarely seen, but their appearance is wonderfully curious. They generally appear about the size of the true sun, not quite so bright, though they are said sometimes to rival their parent luminary in splendour. When there are a number of them they are not equal to each other in brightness. Externally, they are tinged with colours like the rainbow. They are not always round, and have sometimes a long fiery tail opposite the sun, but are paler towards the extremity. They are formed by the reflection of the sun's beams on a cloud.. QUESTIONS.-1. Under what circumstances do we perceive the rainbow? 2. What is a halo? 3 What are parhelia, or mock-suns 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-adistance.] SPECTACLES. 81 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 smaliness, 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 as to see them nearer than with the naked eye, without affeeting 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 |