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OPPENHEIM 31 OPTICAL ILLUSION Great Snare"; "The Master Mummer"; "Anna the Adventuress"; "Mysterious Mr. Sabin"; "The Traitor"; "A Prince of Sinners"; "Mr. Wingrave"; "A Maker of History"; "The Secret"; "Conspira- tors"; "The Missioner"; "Jeanne of the Marshes"; "The Illustrious Prince"; "The Missing Delora"; "The Falling Star"; "Havoc"; "The Lighted Way"; "The Mischief Maker"; "The Game of Liberty"; "People's Man"; "Mr. Grex of Monte Carlo"; "The Double Traitor"; "The Hillman"; "The Wicked Marquis"; "The Curious Quest." E. PHILLIPS OPPENUEIM OPPENHEIM, JAMES, an American novelist and short story writer, bom at St. Paul, Minnesota, in 1882. After studying for a time at Columbia Uni- versity he became in 1905 the superin- tendent of the Hebrew Technical Schools for Girls in New York City. He re- signed this position in 1907 to give him- self entirely to literary work. His best known novels are "Doctor Rast" (1909) ; "Wild Oats" (1910); "Idle Wives" (1914). Besides novels he has written several volumes of poetry such as "War and Laughter" (1916); "The Book of Self" (1917). OPPEB, FREDERICK BURR, Amer- ican artist, born in Madison, 0., 1857; left school at fourteen to work in village newspaper office. Came to New York, worked there in a store, then, having sold some humorous sketches, "Wild Oats," took up illustrating as a profession. Was on art staff of Frank Leslie's three years; with "Puck" eighteen years, which he left to go on the New York "Journal." Has illustrated for Bill Nye, Mark Twain, Dunne (Mr. Dooley) ; is widely known from his "Happy Hooligan," "Al- pbonse and Gaston," and other "funny sheet" sketches. OPPOSITION, in astronomy, the sit- uation of two heavenly bodies when they are diametrically opposed to each other, or when their longitudes differ by 180°. Thus there is always an opposition of sun and moon at every full moon; also the moon, or a planet, is said to be in opposition to the sun when it passes the meridian at midnight. In logic, opposi- tion of judgments is the relation be- tween any two which have the same mat- ter, but a different form, the same sub- ject and predicate, but a different quan- tity, quality or relation. There are five kinds of opposition, viz., contradictory, contrary, inconsistent, subaltern, and subcontrary. In rhetoric, a figure whereby two things are joined which seem incompatible. OPSONIN. A substance which is be- lieved to exist in the blood, and whose function it is to stimulate the phagocites in their attack upon harmful bacteria, possibly by rendering these foreign micro-organisms more readily absorbable. The opsonic index is the ratio between the power of absorbing bacteria pos- sessed by the blood under healthy condi- tions and that possessed under pathologi- cal conditions. In other words, the higher the opsonic index, the greater is the resistance of the body to disease. It has been found that the opsonic index can in certain diseases be raised by in- jecting into the blood dead bacteria of the species producing the disease. OPTATIVE, in grammar, that form of the verb in which wish or desire is expressed, existing in the Greek and some other languages, its force being conveyed in English by such circumlocutions, as "may I," "would that he," etc. OPTICAL ILLUSION. An object ap- pears large or small, near or distant, ac- cording as the rays from its opposite borders meeting at the eye form a large or a small angle ; when the angle is large, the object is either large or near; when small, the object must be small or dis- tant. Experience alone enables us to de- cide whether an object of large apparent size is so on account of its real size, or of its proximity ; and our decision is arrived at by a comparison of the object in post-