DISSIPATION

, in Physics, a gradual, slow, insensible loss or consumption of the minute parts of a body; or, more properly, the flux by which they fly off and are lost. See Effluvia.

Circle of Dissipation, or Aberration, in Optics, denotes that circular space upon the retina of the eye, which is occupied by the rays of each pencil in indistinct vision: thus, if the distance of the object, or the constitution of the eye, be such, that the image falls beyond the retina, as when objects are too near; or before the retina, when the rays have not a sufficient divergency, the rays of a pencil, instead of being collected into a central point, will be dissipated over this circular space: and, all other circumstances being alike, this circle will be greater or less, according to the distances from the retina of the foci of refracted rays. But this circle causes no perceptible difference in the distinctness of vision, unless it exceed a certain magnitude; as soon as that is the case, we begin to perceive an indistinct- | ness, which increases as that circle increases, till at length the object is lost in confusion. This circle is also greater or less, according to the greater or less magnitude of the visible object: and though it be not easy to assign the diameter of the said circle, it seems very probable that vision continues distinct for all such distances, or so long as these circles, or the pencils of light from them, do not touch one another upon the retina; and the indistinctness begins when the said circles begin to interfere. It has been often observed, that a precise union of the respective rays upon the retina, is not necessary to distinct vision; but the sirst author who ascertained the fact beyond all doubt, was Dr. Jurin. See a variety of observations and experiments on this subject, in his Essay on Distinct and Indistinct Vision, in Smith's Optics, Appendix. In the Philos. Trans. for 1789, pa. 256, is an excellent paper on this subject by Dr. Maskelyne; in which he computes the diameter of the circle of dissipation at .002667 of an inch, making it answer to an external angle of 15°, which he shews is very compatible with distinct vision. See also Moon, and Vision.

Radius of Dissipation, is the radius of the circle of dissipation.