2. Lay a shilling, or any piece of money, at the bottom of a bason; then withdraw from the bason, till you lose sight of the shilling; fill the bason nearly with water, and the shilling will be seen very plainly, though you are at the same distance from it. 3. Place a stick over a bason which is filled with water; then reflect the sun’s rays, so that they may fall perpendicularly on the surface of the water; the shadow of the stick will fall on the same place, whether the vessel be empty or full. What has been said of water, may be applied to any transparent medium, only the power of refraction is greater in some than in others. It is from this wonderful property, that we derive all the curious effects of glass, which make it the subject of optics. It is to this we owe the powers of the microscope and the telescope. To produce these effects, pieces of glass are formed into given figures, which, when so formed, are called lenses. The six following figures are those which are most in use for optical purposes. 1. A PLANE GLASS, one that is flat on each side, and of an equal thickness throughout. F, Fig. 13. Plate I. 2. A DOUBLE CONVEX GLASS, one that is more elevated towards the middle than the edge. B, Fig. 13. Plate I. 3. A DOUBLE CONCAVE is hollow on both sides, or thinner in the middle than at the edges. D, Fig. 13. Plate I. 4. A PLANO CONVEX, flat on one side, and convex on the other. A, Fig. 13. Plate I. 5. A PLANO CONCAVE, flat on one side, and concave on the other. C, Fig. 13. Plate I. 6. A MENISCUS, convex on one side, concave on the other. E, Fig. 13. Plate I. It has been already observed, that light proceeds invariably from a luminous body, in strait lines, without the least deviation; but if it happen to pass from one medium to another, it always leaves the direction it had before, and assumes a new one. After having taken this new direction, it proceeds in a strait line, till it meets with a different medium, which again turns it out of its course. A ray of light passing obliquely through a plane glass, will go out in the same direction it entered, though not precisely in the same line. The ray C D, Fig.4. Plate I. falling obliquely upon the surface of the plane glass A B, will be refracted towards the glass in the direction D E; but when it comes to E, it will be as much refracted the contrary way. If the ray of light had fallen perpendicularly on the surface of the plane glass, it would have passed through it in a strait line, and not have been refracted at all.