OBSCURE RAYS.—Invisible rays above and below those of the visible spectrum. (See Spectrum.) OIL.—The term oil is generally applied to all neutral fatty substances which are liquid at )rdinary temperatures. They may be divided into two great classes,* fixed oils and essential oils.
• Rodwell's " Directory of Science." Oils are inflammable either at the ordinary temperature or when heated. The fixed oils are not volatile without decomposition. Some of them oxidize when exposed to the air, and dry to a caoutchouc-like substance. The essential oils are of a peculiar pungent odor, distill without decomposition, and are very inflammable.
OIL COLORING.—Photographs may be colored in oils if first coated with japanner's quick gold size. The following excellent instructions for tinting in oils are given by a correspondent in Photography* For tinting pictures in oil the following colors will be required : White, Naples-yellow, yellow orchre, raw sienna, burnt sienna, mars orange, light red, vermillion, pink madder, crimson lake, Indian red, raw umber, burnt umber, terra verte, emerald green, ultra marine, Prussian blue, ivory black, and whatever other colors the particular draperies and back ground may require. Payne's gray is also useful. Use sufficient megilp to render the colors thin and transparent, and lay the colors on with as little disturbance as possible to secure their purity. Commence by carrying a warm tint of light red and burnt umber over the darkest shadows ; use white, terra verte and Indian red for the lighter shadows, white and terra verte for blending the high lights. The following are the best combinations for various complexions : White, Naples-yellow, and vermillion ; the same, with an addition of light red, white and pink madder, with a little vermillion for the tint on cheek. If these colors are mixed in small quantities those most suitable for the model can be easily selected. The shadows may be glazed over with a mixture of white, light red, and emerald green, or white, Indian red, ultramarine, and raw umber, the complexion of the person indicating the choice. The high lights should be touched in with white and Naples-yellow graduating into the local color. Strengthen the
nostrils and the lines of the eyelids, and that separating the lips with Indian red. Carry a line of brown or indigo, as may be required round the iris of the eye, put in the local tint, the reflected light and the pupil, remembering the part of the eye called white is really grayish, more or less light, according to the position of the eyes and length of eyelashes. The eyebrows and hair should be glazed with a suitable brown, keeping the former soft, transparent, and hairlike, and the divisions of the hair well but not too strongly defined, and the hair transparent where it meets the brow. The high lights of the hair will be bluish, being colder by contrast as the hair is dark or fair. Use Payne's gray mixed with shadow tints to blend the hair and face. Black dresses or coats should be glazed with a warm transparent black, into which strengthen the lights with tints of black and white mixed, and deepen the shadows with Vandyke brown and a little lake. Should the hands require painting they should be done with the same tint as face, with the addition of a rosy tinge on the knuckles and tips of fingers, and the divisions strengthened with a warm shadow tint. All draperies may be done as in case of black with such colors as the fabric may require, remembering always where the lights are cold the shadows must be warm. For lace or linen use white and blue-black, white, black, and burnt umber, with white for the lights. For gold ornaments use yellow ochre or Naples-yellow, yellow ochre and raw umber, and burnt sienna and raw umber." OILED NEGATIVES.—Negatives made upon paper used as a substitute for glass are usu ally oiled or otherwise treated to render them more translucent, and consequently to print with greater rapidity Oiled negatives require to be kept in oiled or waxed paper. (See Paper Negatives and Translucent.) OILED PAPER.—Paper treated with oil. (See Translucent.) A paper of this kind is often useful for photographic purposes—for instance, where an air-proof packing is required. Oiled-paper negatives should be kept in oiled or waxed paper.