Reticule, in Astronomy, a contrivance for measuring very nicely the quantity of eclipses, &c. This instrument, which was introduced by the Academy of Sciences at Paris, is a little frame composed of 13 fine silken threads, parallel to, and at equal distances 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. The diameter of the sun or moon is of consequence thus seen divided into 12 equal parts or digits; so that, in order to ascertain the quantity of the eclipse, there is nothing more to do than 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 perfectly represents the phases of the eclipse.

But it is obvious that whether the Reticule be square or circular, it should 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. Another imperfection in the Reticule is, that its its magnitude is determined by that of the image in the focus, and of course it will only fit one particular magnitude.

But a remedy for these inconveniences has been found out by M. de la Hire, who contrived that the same Reticule may serve for all telescopes, and all magnitudes of the luminary in the same eclipse. Two object glasses applied against each other, having a common focus, and these forming an image of a certain magnitude, this image will increase in proportion as the distance between the two glasses is increased, as far as to a certain limit. If therefore a Reticule be taken of such a magnitude, as just to comprehend the greatest diameter the sun or moon can ever have in the common focus of two object glasses applied to each other, it is only necessary to remove them from each other, as the star comes to have a less diameter, to have the image still exactly comprehended in the same Reticule.

As the silken threads are apt to deviate from the parallelism, &c. by the different temperature of the air, another improvement is, to make the Reticule of a thin looking glass, by drawing lines or circles upon it with the fine point of a diamond.