, or Reticule, in Astronomy, a contrivance for measuring very nicely the quantity of eclipses, &c.

This instrument, introduced some years since by the Paris Acad. of Sciences, is a little frame, consisting of 13 sine silken 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 6 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.

But M. de la Hire has found a remedy for all these inconveniences, and contrived that the same Reticule shall serve for all telescopes, and all magnitudes of the luminary in the same eclipse. The principle upon which his invention is founded, is that 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, there needs nothing but 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.

Farther, as the silken threads are subject to swerve 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. See MICROMETER.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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