unscrew the tube which contains the eye glass and micrometer, from the rest of the telescope, and, looking through the eye glass, place the micrometer where the divisions of it may appear most distinct to his eye. The mother of pearl scale may be about the 24th part of an inch broad; its length is determined by the aperture of the diaphragm; its thickness that of writing paper. The divisions on it maybe the 200th of an inch, which may reach from one edge of the scale to about the middle; and every fifth and tenth division may be a little longer, the tenths going quite across. When the telescope does not magnify above 30 times, the divisions need not be so minute. For the sake of those not conversant in trigonometry, the following is an easy method of determining the value of the divisions on the scale. Mark upon a wall or other place, the length of 6 inches; then place the telescope before it so that the 6 inches be at right angles to it, and exactly 57 feet 3¹⁄₂ inches distant from the object glass of the telescope. This done, look through the telescope, and observe how many divisions of the micrometer are equal to it, and that same number of divisions will be equal to half a degree, or 30′; and this is all that need be done to ascertain the value of the scale. The reason on which it is founded is, that an extension of six inches at the distance of 57 feet, 3¹⁄₂ inches, subtends an angle of 30′, as is easily calculated by trigonometry. To save the trouble of calculation, a scale may be made requiring only inspection. Thus, draw a line equal to the diameter of the field of the telescope, and divide its under side into the same number of parts as are on your micrometric scale, and, by the above operation on the wall, having determined the value of 30′, which we will suppose to correspond with 16 divisions on the scale, mark 30′ on the opposite side of the line, opposite 16 on the lower; 15 opposite 8, and so on. By the following table the results may be ascertained by inspection only: thus, suppose an extension of 1 foot is found by the table to subtend an angle of 22′, the distance will be 156.2: and suppose at the distance of 171.8 an object subtends an angle of 20′, its height will be found to be 1 foot; or, suppose an object of 6 feet high to subtend an angle of 20′, the distance is 1030.8, by multiplying 171.8 by 6. _Table of Angles subtended by 1 Foot, at different Distances._