suggested. Fig. 3 contains a representation of a wooden building, on the top of which is a large convex lens HI, about 10 or 12 feet focal distance. At half a right angle to this lens is a plain speculum, by which the rays of light from the objects O are reflected downwards through the lens, which forms a picture of all the objects before the speculum, on a round white table, T, in all their colours, motions, and proportions. If the speculum be made to revolve, the whole of the surrounding landscape may be successively depicted on the table. When the lens is of a long focal distance, as from 10 to 15 or 20 feet, it produces a pretty powerful telescopical effect, so that objects may be distinctly perceived at a considerable distance, and individuals recognised on the picture at the distance of a mile or more. Wherever there are objects in motion, such as ships sailing, birds flying, smoke ascending, crowds of people moving to and fro, or boys and girls engaged in their amusements; this exhibition always affords a high degree of satisfaction. It might occasionally be used, not only as an illustration of optical principles, but also as a reward for diligence and good behaviour. In connection with the above, representations might be given of natural and artificial objects as exhibited by the phantasmagoria. Discarding the ridiculous and childish figures which were formerly used in the common magic lanterns, opticians have now constructed sliders which exhibit representations of the telescopic appearances of the heavenly bodies, the different constellations, the motions of the earth and moon, and various objects connectea with botany, mineralogy, and zoology; and such objects, wher exhibited in this manner, are calculated to produce both instruction and amusement. The solar microscope in particular, (or the oxy-hydrogen, if it can be procured,) should be occasionally exhibited to the young, to convey to them some ideas of the wonderful minuteness of the atoms of matter, and the admirable mechanism displayed in the structure of vegetables and the bodies of animals, particularly in those myriads of animalcula which are invisible to the unassisted eye. Such animalculæ may be procured almost at any season, but particularly during the summer months, by infusing in separate open vessels, small bits of grass or hay, leaves of flowers, or other vegetable substances, when, after a week or ten days, animalculæ of different kinds, according to the nature of the substances infused, will be perceived in vast numbers, by the aid of the microscope, in every drop of the infusion. A compound microscope is perhaps as good an instrument as any other for giving a steady and satisfactory view of such objects; and the only objection to its use for a school is, that only one individual can see the object at a time. When a teacher is not furnished with an instrument of this kind fitted up in the usual way, he may, with little trouble, construct a com pound microscope, by means of the eye-piece of a common pocket achromatic telescope, which may be purchased for one guinea, or less. The eye-pieces of such telescopes contain four glasses arranged on a principle somewhat similar to that of the glasses of a compound microscope. If we screw off one of these eye-pieces, and look through it in the usual way, holding the object end about a quarter of an inch distant from any small object, such as the letters of a printed book, it will appear magnified about ten or twelve times in length and breadth; remove from the tube the third glass from the eye, which is the second from the object, and look through it in the same manner, holding it more than an inch distant from the object, and it will appear magnified more than twenty times in diameter, or above 400 times in surface. If, by means of small pasteboard tubes, or any other contrivance, we attach the glass that was taken out of the outside of the objectglass of the eye-piece, so as to be nearly close to it, we shall have a magnifying power of nearly forty times; or, if we substitute for these two object-glasses a single glass of about a halfinch focal distance, we shall form a pretty good compound microscope, magnifying above forty times in diameter, and 1600 times in surface, which will afford very pleasing views of various ob jects in the animal and vegetable kingdoms. The magnifying powers now stated will differ somewhat in different eye-pieces, according to their lengths and the focal distances of the glasses of which they are composed. The tube of the eye-piece thus arranged, may be occasionally fitted into a pasteboard tube supported by three pillars, in which it may be moved up or down for adjusting it to distinct vision, and the object placed underneath and properly illuminated. These hints are suggested on the score of economy, for those who have no regular microscopic apparatus. Various amusing experiments besides the above might be exhibited to the young, such as the optical paradox, an instrument through which objects may be seen, although a board or other opaque body be interposed between the eye and the objects-the prism, which, in a dark room, separates the primary colours of the solar rays-the multiplying glass, which makes one object appear as if there were ten, twenty, or thirty-the burning glass, which, by means of the sun's rays, sets on fire dark coloured paper, wood, and other inflammable substances-and optical illusions produced by the various refractions and reflections of light in water, combinations of plane mirrors, and by concave speculums. A concave mirror, about 5 or 6 inches diameter, and 10 or 12 inches focus, which may be procured for about half-a-guinea or 15 shillings, is of great utility for a variety of exhibitions. 1. When held at nearly its focal distance from one's face, it represents it as magnified to a monstrous size. 2. When held in the solar rays, directly opposite the sun, it collects the rays into a focus before it, so as to act as a powerful burning-glass, and in this way a hole may be burned in a thin board. 3. When hung at an elevation of about 5 feet, and a person placed opposite to it, at 6 or 7 feet distant, he will see his image hanging in the air in an inverted position, between him and the mirror, and if he approach a little nearer the mirror, and hold out his hand towards it, the image will appear to do the same, as if about to shake hands, and if he stretch his hand still nearer the mirror, the hand of his image will appear to pass by his hand, and approach nearer his body. 4. Such a mirror is of use in explaining the construction of a reflecting telescope. When it is held opposite to a window, the image of the sash and of the objects without the window will be seen depicted in its focus on a piece of white paper held between it and the window, which represents the manner in which the first image is formed by the great mirror of a reflecting tele. scope;-and the manner in which the small speculum of a Gre gorian reflector forms the second image, may be shown by hold ing the mirror at a little more than its focal distance behind a candle, and throwing its magnified image upon an opposite wall, in the same way as the lens, fig. 1, p. 239, by refraction, produced the enlarged image CD. 5. If a bright fire be made in a large room, and a very smooth, well-polished mahogany table be placed at a considerable distance near the wall, and the concave mirror so placed that the light of the fire may be reflected from the mirror to its focus on the table-a person standing at a distance toward the fire, but not directly in the line between the mirror and the fire, will see an image of the fire upon the table, large and erect, as if the table had been set on fire. Various illusions and deceptions have been produced by means of concave mirrors. Pagan priests are supposed to have rekindled the Vestal fire by this instrument; and with the same instrument, on a large scale, Archimedes is reported to have burned the Roman fleet. When the mirror is concealed from the view of a spectator by certain contrivances, he may be easily deceived ana tantalized with a shadow instead of a substance. He may be made to see a vessel half full of water inverted in the air without losing a drop of its contents. He may be desired to grasp what appears a beautiful flower, and, when he attempts to touch it, it vanishes into air, or a death's-head appears to snap at his fingers. He may be made to behold a terrific spectre suddenly starting up before him, or a person with a drawn sword, as if about to run him through. An exhibition of this kind was some time ago brought before the public, which was effected by a concave mirA man being placed with his head downwards, in the focus of the mirror, an erect image of him was exhibited, while his real person was concealed, and the place of the mirror darkened; the spectators were then directed to take a plate of fruit from his hand, which, in an instant, was dexterously changed for a dagger or some other deadly weapon.-It may not be improper occasionally to exhibit such deceptions to the young, and leave them for some time to ruminate upon them till the proper explanations be given, in order to induce them to use their rational powers in reflecting on the subject, and particularly to teach them to investigate the causes of every appearance that may seem mysterious or inexplicable, and not to ascribe to occult or supernatural causes what may be explained by an investigation of the established laws of nature; and to guard them against drawing rash or unfounded conclusions from any subject or phenomenon which they have not thoroughly explored, or do not fully comprehend. ror. Having enlarged much farther than I originally intended on the preceding departments of Natural Philosophy, I have no space left for suggesting any hints in relation to electricity, galvanism, and magnetism. If the teacher is possessed of an electrical machine and a galvanic apparatus, and is acquainted with his subject, he has it in his power to exhibit a great variety of very striking experiments which can never fail to arrest the attention of the juvenile mind, and prepare it for entering on explanations of some of the sublimest phenomena of nature. But without these instruments very few experiments of any degree of interest can be performed in relation to these subjects. The illustration of the phenomena of magnetism requires no expensive apparatus. Two or three small, and as many large bar magnets a large horse-shoe magnet, a magnetic compass, and a few needles, pieces of iron, and steel filings, may be sufficient for illustrating the prominent facts in relation to this department of philosophy. But as I have already thrown out a few hints on this subject in the lesson on the Sagacious Swan,* it would be needless to enlarge. My only reason for suggesting the above hints and experiments is, to show that any teacher, at a very small expense, may have it in his power to illustrate, in a pleasing manner, many of the most interesting and practical truths connected with natural philosophy. Most of the apparatus alluded to above could be procured for two or three pounds, provided the experimenter apply his hands and construct a portion of it himself, which he can easily do when the materials are provided. In regard to philosophical apparatus of every description, were there a general demand for it from all classes of the community, it might be afforded for less than one half the price now charged for it, as certain portions of it might be constructed of cheaper materials than are now used; as elegance in such instruments is not always necessary for use; as competition would reduce their price to the lowest rate, and as there would be no necessity for great profits when the manufacturers were certain of a quick and extensive sale. Chemistry.-Chemistry, in it present improved state, is a science so interesting and useful, so intimately connected with the knowledge of nature, the improvement of the useful arts, and with every branch of physical and practical science, that an outline, at least, of its leading principles and facts should be communicated to all classes of the young. The distinguishing properties of the simple substances, such as oxygen, nitrogen, carbon, hydrogen, chlorine, iodine, sulphur, and phosphorus-particularly oxygen, nitrogen, carbon, and carburetted hydrogen, should be * Sec page 154. |