Explanation of the two Scales on Dr. Franklin’s Prismatic Telescope or Micrometer One Scale gives the angle subtended by any object, measured by the Micrometer. The larger Divisions are minutes of a degree and the smaller divisions 10 seconds each. It seems to be particularly adapted to the apparent Diameters of the Sun and Moon.

The other Scale is contrived for measuring the Distance of an object from the Observer. Thus, if the magnitude of a distant object be known and you measure it with the Micrometer, the Index will point to a number by which multiply that Diameter and you will have the distance of the Object in like measure.

But if the magnitude of the object be unknown two measures must be taken at different distances, and that difference noted. Then as the difference of the numbers pointed at by the Index, is to the difference of distances, so is either number to its corresponding distance.

The first Scale with a little Calculation will answer both purposes by far the best, as the other from the nature of it does not admit of an accurate subdivision. If 3438 be divided by the minutes or fractional parts given on the first scale it will give the corresponding number on the other Scale. Note. 3438 is the lenghth of a Base on which a parpendicular of 1. subtends an angle of one minute.