1. The word optician is applicable to persons who are particularly skilled in the science of vision, but especially to those who devote their attention to the manufacture of optical instruments, such as the spectacles, the camera obscura, the magic lantern, the telescope, the microscope, and the quadrant.
2. Light is an emanation from the sun and other luminous bodies, and is that substance 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 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 free passage of light, is called a medium. 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 which pass through it. These are of different shapes; and they have, therefore, received different appellations. A plano-convex 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.
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.
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. All the light reflected from the latter 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 color, is placed, the image of the object will be seen upon it.