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154 ^ETHRIOSCOPE AETIUS Geschichte der Aesthetik (Vienna, 1858) ; Vi- scher, Aesthetik, oder Wissenschaft des Sehonen (Reutlingen, 1846-'57); Zeising, Aesthetische Forschungen (Frankfort, 1855) ; Kostlin, Aes- thetik (Tubingen, 1863); Gottfried Semper, Der Stil in den technischen und tektonischen Kunsten, oder pralctische Aesthetik (Frankfort, 1860-'63) ; J. Dippel, Handbuch der JEsthetik, &c. (Regensburg, 1871). Among Englishmen, Dugald Stewart, Hutcheson, Alison, Jeffrey, and Payne Knight have written on aesthetics; Burke wrote " A Philosophical 'Inquiry into the Ori- gin of our Ideas of the Sublime and Beauti- ful," but the work has little depth. The op- posing theories of these older writers have long ceased to attract attention; and, as in Germany, later works on the subject have fol- lowed the method d posteriori. Sir William Hamilton, it is true, in his " Lectures on Meta- physics," considers in the abstract the philoso- phy of the beautiful ; but other recent writers, like Ruskin, whose aesthetical works are the most voluminous, treat of beauty in form and color. Two recent American works may be also noticed : "The Science of ^Esthetics," by Henry N. Day (New Haven, 1872), an<l " Lec- tures on ^Esthetics," by Professor John Bascom (New York, 1872). One of the best modern writers on aesthetics is the French critic Hippo- lyte Adolphe Taine, whose principal works on art form a series of essays on the productions of almost every school. See his Philosophic de Tart (Paris, 1865), Philosophic de Vart en Italic (18(56), Voyage en Italic (1866), IS Ideal dans Vart (1867), Philosophic de Part dans leg Pays-Bas (1868), &c., translated into Eng- lish by J. Durand (New York, 1866-'70). Among older French writers on aesthetical subjects are Cousin (Le vrai, le lean et le bori) and Jouffroy (Cours d'esthetique, Paris, 1842). JCTIIRIOSCOPE (Gr. aWpio^ clear, and oiumelv, to observe), an instrument invented by Sir John Leslie for measuring the relative degrees of cold produced by the radiation toward a clear sky. In a metallic cup standing upon a tall hollow pedestal, a differential thermometer is placed in such a manner that one of its bulbs is in the focus of the paraboloid formed by the cavity of the cup, and the other bulb is beyond the hol- low of the cup. The interior of the cup is highly polished, and is kept covered by a plate of metal, and only opened when an observation is to be made. As the second bulb is out of the cup, it is not affected by the radiation, the ac- tion of which is concentrated upon the first bulb. The contraction of the air in this bulb by its sudden exposure to a clear sky causes the liquid in the stem to rise. The figure rep- resents a vertical section of the aethrioscope. A B D is the parabolic cup, of which the inside is plated with silver and well polished ; in its focus one of the bulbs F of the differential ther- mometer F H T is placed ; the other bulb T is outside the cup. Any difference of expansion between the air in the two bulbs is made visible by the motion of a short column of fluid in the tube, and read off on the scale in H. The sup- port E K, with a hinge in I, connects it with the heavy footpiece G, so that it may be inclined in different positions, and directed toward differ- ent portions of the sky. Its inclination should never be made such as to expose the thermo- metric bulb in the focus F to terrestrial ob- jects above the horizontal line H H, as these would either reflect or radiate terrestrial heat, and so entirely or partially annul the cooling of the bulb F by its own 7-adiation. The pol- ished surface of the cup, like all such surfaces, cannot radiate its own heat, but only reflect that of the bulb F ; it forms thus a barrier be- tween the earth and the cup, impenetrable to terrestrial heat. Leslie could not interpret the indications of this instrument satisfactorily. Not only a passing cloud checked the loss of heat, but, he says, "sometimes under a fine blue sky the rothrioscope will indicate a cold of 50 mil- lesimal degrees, while on other days, when the air seems equally bright, the effect is scarcely 30." It has only recently become known that such differences are due to the presence of aqueous vapors in the air, totally invisible to the eye, but which, being more or less opaque to the feeble rays of radiant heat, screen the bulb and reflecting cup of the aethrioscope against loss of heat by radiation, while a dry atmos- phere admits this radiation to pass, and more freely in proportion as the air is more dry. The aethrioscope is therefore at the present day used as a hydrometer to determine the amount of invisible moisture present in the upper inac- cessible strata of our atmosphere. KTIO V a famous Greek painter, supposed to have lived in the first half of the 2d century. He was distinguished for the beauty of his coloring, and esteemed the first painter of his time. Lucian gives a description of a very fine painting by him, representing the nuptials of Alexander and Roxana, which was displayed at the Olympic games. AETIIS, surnamed the Atheist, from his sup- posed denial of the God of revelation, an orien- tal heresiarch, born in Antioch, died in Con- stantinople, A. D. 367. In early life he was