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OPTIC NERVE 32 OPTOMETRY tion with other common objects, such as trees, houses, etc. The same is, of course, true of apparently small objects. But when all means for comparison are removed our judgment is at fault. Sim- ilarly, we erroneously infer spherical solids at a distance to be flat disks, and a man in a white habit seems larger than he would if he wore a dark dress. Illu- sions are also produced by external causes. The persistence of impressions on the retina for about one-sixth of a second after the object which produced the im- pression has been removed produces an- other class of illusions. Common exam- ples of this are the illuminated circle formed by the rapid revolution of an ig- nited carbon point, piece of red-hot iron, or other luminous body. Another form of illusion is produced to a person who is seated in a vehicle in motion. The illu- sion is most complete when the attention is riveted on an object several yards off; this object then appears to be a center round which all the other objects re- volve, those between the observer and the object moving backward, and those be- yond the object moving forward. Other illusions arise from a disordered state of the organs of vision: e. g., the seeing of things double or movable, or of a color different from the true one. OPTIC NERVE, in anatomy, the nerve of sight, proceeding from the optic lobes or corpora quadrige^nina to the eye, terminating in an expansion called the retina.' The inner portion of the fibers of the two optic nerves decussates at the commissure, passing to the opposite eye, while the outer portion continues its course to the eye of the same side, which has been supposed to assist in the _pro- duction of single vision, although it is more probable that the latter is the result of a mental act. The closest relations exist between the optic nerve, its disk, the retina, and the choroid, as regards the cerebral and intraocular circulation, particularly seen in the course of cere- bro-spinal disease. OPTIC NEURITIS. Condition of the optic nerve brought about by inflamma- tion showing two distinct phases, namely, retrobulbar neuritis in which the nerve fibers behind the eye are affected, and papillitis, where the seat of inflammation is in the optic dtsk. The condition is often the result of brain disease, acute fevers, and syphilis, affecting the vision in common with other subjective symp- toms. OPTICS, in physics, that branch of knowledge which treats of the properties of light and of vision as performed by the human eye. The modern division of the science is into: 1. Sources of light; 2. Transmission, velocity, and intensity of light; 3. Reflection of light — mirrors; 4. Single refraction — lenses; 5. Disper- sion and achromatism; 6. Optical instru- ment; 7. The eye considered as an opti- cal instrument; 8. Phosphorescence and fluorescence; and 9. Double refraction, interference, and polarization. OPTIMISM, in philosophy, the name given to the view propounded in the "Thedicee" of Leibnitz that this world, as the work of God, must be the best among all possible worlds. In general, the tendency to take the most hopeful view of matters in general; the belief that the world is growing better. OPTOCHIN. Ethylhydrocupreine. C1.H.0N2OH. C.C2H5. A derivative of cupreine, which occurs, together with quinine in the bark of Remijia peduncu- lata. It is a white, or faintly yellow powder, with a bitter taste. Almost in- soluble in water. Possesses properties similar to those of quinine, and is used in the treatment of certain infections of the cornea of the eye. Internally the drug is uncertain in its action, and its administration has sometimes been fol- lowed by deafness and blindness, either temporary or permanent. For treating the eye a 2 per cent, solution of optochin hydrochloride is recommended. OPTOGrRAPHY, in optics, the tem- porary retention in certain cases of an image, that of the last person or thing seen, on the retina of the eye when a man or a beast dies. OPTOMETRY. The science of meas- uring the vision and strength of the eye without the dilation of the pupil by the use of drugs, and the selection of proper lenses to correct defective vision. The use of drugs to dilate the pupils while making an examination is not legal, except by a registered physician — an oculist. The profession of optometry has been established by those opticians who de- sired to place their work on a higher plane than that occupied by the ordinary seller of optical goods. An organization was formed in 1904, and committees ap- pointed who worked for legislative regu- lation of the practice of their profession. Minnesota was the first state to ac- knowledge the new profession, but now almost every state, as well as the Cana- dian provinces, recognizes and regulates it by legislative action. The general requirements for' the prac- tice of optometry are a thorough knowl-