DEVIATION. When a ray of light is refracted or reflected at the surface of any medium, and follows a different course to that which it had before, the angle through which it is turned out of its original path is called its deviation. For instance, if QR be a ray of light incident on the siulace of a medium at R, and RS its path after refraction or reflection at R, then, if Q R be produced to T, the angle T RS is the deviation of the ray.
The terra "deviation" must not be confound.ed with "aberration." DEw ; Dicw Pot/qr.—When a glass of iced water is brought into a warm room in summer, dew is deposited upon it, from the condens ation of the aqueous vapour in the air by the oold glass. For the same reason, if' a cold g,lass lens be taken into warm air, it becomes covered with dew, and this interferes with the production of a sharp, dear picture. Sometimes a patch of dew on the middle of a lens will produce a patch of indistinctness and fogginess in the centre of the picture. It sometimes happens also in the paper processes, that
when the excited paper is placed in the slide behind a glass, inste,ad of being attached to the front of it, the cold glass becomes covered with dew on being taken into the warm air, which of course ruins the picture if expose,d before the dew has evaporated. In the .
daguerreotype process also, if a cold plate is placed in summer over a warm solution of bromine, or if a cold iodized plate is taken into the warm air, dew condenses upon it, and this int,erferes with the production of a fine picture.
The dew point is the temperature at which dew begins to be de posited ; it varies according to circumstances. The temperature of the lens, at the time of using it, should be above the dew point.