RETICULA, or RETICULE, in astronomy, a con-
trivance for measuring very nicely the quantity of eclip-
ses, &c. This instrument, introduced some years since
by the Paris Academy of Sciences, is a little frame,
consisting of 13 fine silken threads, parallel to,
and equidistant from, each other, placed in the focus of ob-
ject-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 lu-
minous. 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 concen-
tric equidistant circles, which represents the phases of
the eclipse perfectly. But it is evident that the reti-
cule, 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; other-
wise 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 magni-
tude is determined by that of the image in the focus;
and of consequence it will only fit one certain magni-
tude. See MICROMETER, Encycl.