or **RETICULI**, in astronomy, a contrivance for measuring very nicely the quantity of eclipses, &c. This instrument, introduced some years since by the Paris Academy of Sciences, is a little frame, consisting of 13 fine filken threads, parallel to, and equidistant from, each other, placed in the focus of object-glasses of telescopes; that is, in the place where the image of the luminary is painted in its full extent. Consequently the diameter of the sun or moon is thus seen divided into 12 equal parts or digits: so that, to find the quantity of the eclipse, there is nothing to do but to number the parts that are dark, or that are luminous. As a square reticule is only proper for the diameter of the luminary, not for the circumference of it, it is sometimes made circular, by drawing six concentric equidistant circles, which represents the phases of the eclipse perfectly. But it is evident that the reticule, whether square or circular, ought to be perfectly equal to the diameter or circumference of the sun or star, such as it appears in the focus of the glass; otherwise the division cannot be just. Now this is no easy matter to effect, because the apparent diameter of the sun and moon differs in each eclipse; nay, that of the moon differs from itself in the progress of the same eclipse. Another imperfection in the reticule is, that its magnitude is determined by that of the image in the focus; and of consequence it will only fit one certain magnitude. See MICROMETER, Encycl.