Aethrioscope (Gr. clear, and 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 hollow 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 action 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 represents a vertical section of the Aethrioscope. A B C 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 thermometer 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 II. The support 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 different portions of the sky.

Its inclination should never be made such as to expose the thermo-metric bulb in the focus F to terrestrial objects 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 radiation. The polished 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 between 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 rethrioscope will indicate a cold of 50 millesimal 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 sethrioscope against loss of heat by radiation, while a dry atmosphere admits this radiation to pass, and more freely in proportion as the air is more dry.

The rethrioscope is therefore at the present day used as a hydrometer to determine the amount of invisible moisture present in the upper inaccessible strata of our atmosphere.