ILLUMINATING LENS.—A large lens, which has for its purpose the concentration of the light from the sun or a lamp at the focus, is sometimes termed an illuminating lens. Condenser is, however, the more general term.
ILLUMINATION.—The illumination of a surface varies inversely as the square of its dis tance from the source of light. The same quantity of light which falls upon a certain area at a distance of one foot from the illuminant will become extended over four times that area if the distance be increased to two feet, nine times at a distance of three feet, i6 times at a distance of four feet, and so on. If, therefore, in contact printing or enlarging, an exposure of one second is required at a distance of one foot from the illuminator, four seconds will be required at two feet, and so on.
111A0E.—An optical term, denoting the representation of objects formed by concourse of the pencils of rays proceeding from the various points of the object which it represents. In optics, a luminous body is considered to be an assemblage of luminous points, from each of which a pencil of light proceeds, without interfering with each other. When an image is formed in the focus of a lens, and can be received on a screen, as, for example, at the focus of a convex lens it is termed real or positive. When, however, it is not formed by the actual union of rays in a focus, but only apparently so, and cannot be received on a screen, it is termed a virtual image. The following useful remarks regarding this subject are from Sir T. Longmore's optical manual: " An image is erect when the object and image lie on the same side of the center of the lens; is inverted when the object and image lie on the opposite side of the center. The retinal image of an object situated in front of the eye is an example of a real and inverted image. The diameters
of an object and its image are directly as their distances from the center of the lens; as they separate from this point, the farther of either is, the greater its proportionate size. (Conjugate Foci.) When an object is placed between a convex lens and its principal focus, an eye on the other side of the lens sees a virtual image of the object erect, magnified on the same side of the lens as, but at a greater distance from it than, the object.
When an object is placed in front of a concave lens, an eye on the other side of the lens sees an image of the object, which is virtually erect, and diminished on the same side of the lens as, and nearer to it than, the object. The image is diminished when the distance between the lens and the object is increased, but when the distance of the object is a large multiple of the focal length of the lens, further increase of its distance does not appreciably alter the distance of the image, or consequently its size.
The image formed by a lens on the screen of the camera is a real image, and its distinct ness will depend upon the accuracy with which the rays of each pencil are brought to a focus. i When an object is placed at a distance in front of the lens an image is formed on the ground glass screen of the camera smaller than the object itself. But if the object be brought nearer the lens the image will increase in size until it becomes equal to, and afterwards larger than, the original. The focus will, at the same time, recede to a greater distance from the lens. (See also Optics, Vision, Light, etc.) IMAGE, LATENT.—See Latent Image.