From this construction we see, that a person at a distance of ten feet may govern the two lucid points, so as to bring them into any required position, south or north, preceding or following, from 0° to 90°, by using the handle P, and also to any distance from of an inch to 5 or 6 and 20 inches, by means of the handle D. If any reflection or appearance of light should be left from the top or sides of the lamps, a temporary screen, consisting of a long piece of pasteboard, or a wire frame covered with black cloth, of the length of the whole arm, and of any required breadth, with a slit of half an inch broad in the middle, may be affixed to the arm by four bent wires, projecting an inch or two before the lamps, situa ted so that the moveable lucid point may pass along the opening left for that purpose.
Plate CCCLXXVI. Fig. 15. represents part of the arm L, of a larger size ; 8 the slider ; m the pulley, over which the chord x / y z is returned towards the centre ; v the other chord going to the pulley n of Fig. 14 ; R the brass piece moveable on the pin c, to keep the lamp upright. At R is a wire rivetted to the brass piece, on which is held the lamp by a nut and screw. Fig. 16, 17. represent the lamps a, b, with the sliding doors open, to show the situation of the wicks. to is the leaden weight, with a hole in it d, through which the wire R of Fig. 15. is to be passed, when the lamp is to be fastened to the slider 8. Fig. 18. represents the lamp a with the sliding door shut ; / the lucid point ; i k the openings at the top, and 8 at the sides for the admission of air.
The ingenious artist will soon perceive that the mo tions of this micrometer are capable of great improve ment by the application of wheels and pinions, and other well known mechanical resources ; but, as the principal object is only to be able to adjust the two lucid points to the required position and distance, and to keep them there for a few minutes, while the observer goes to mea sure their distance, k will not be necessary to say more on the subject.
Sir William Herschel next proceeds to show the ap plication of this instrument. It is well known to opti cians, and others, who have been in the habit of using optical instruments, that we can with one eye look into a microscope or telescope, and see an object much mag nified, while the naked eye may sec a scale on which the magnified picture is thrown. In this manner Sir NV. Herschel generally determined the power of his tele scopes; and any one who has acquired a facility of taking such observations will very seldom mistake so much as one in fifty in determining the power of an instrument, and that degree of exactness is fully sufficient for the purpose.
The Newtonian form is admirably adapted to the use of this micrometer ; for the observer stands always erect, and looks in a horizontal direction, though the telescope should be elevated to the zenith. Besides, his face being turned away from the object to which his telescope is directed, this micrometer may be placed very conveniently, without causing the least obstruction to the view : therefore, in using this instrument, it is put at ten feet distance from the left eye, in a line perpen dicular to the tube of the telescope, and the moveable board raised to such a height, that the lucid point of the central lamp may be on a level with the eye. The
handles, lifted up, are passed through two loops fastened to the tube, just by the observer, so as to be ready for his use. It should be observed, that the end of the tube is cut away, so as to leave the left eye entirely free to see the whole micrometer.
Having now directed the telescope to a double star, it is viewed with the right eye, and at the same time with the left see it projected on the micrometer ; then, by the handle P, which commands the position of the arm, it is raised or depressed so as to bring the two lucid points to a similar situation with the two stars; and, by the handle D, the moveable lucid point is approached or removed to the same distance of the two stars, so that the two lucid points may be exactly covered by, or coincide with the stars. A little practice in this busi ness soon makes it easy, especially to one who has al ready been used to look with both eyes open. • What remains to be done is very simple. With a proper rule, divided into inches and 40th parts, the dis tance of the lucid points is taken, which may be done to the greatest nicety, because, as observed before, the little holes are made with the point of a very fine needle. The measure thus obtained is the tangent of the mag nified angle under which the stars are seen, to a radius of 10 feet ; therefore, the angle being found, and di vided by the power of the telescope, gives the real an gular distance of the centres of a double star. For in stance, Sept. 25, 1781, Sir William Herschel measured Herculis with this instrument. Having caused the two lucid points to coincide exactly with the stars centre on centre, he found the radius, or distance of the central lamp from the eye, 10 feet 4.15 inches ; the tangent or distance of the two lucid points 50.6 fortieth parts of an inch ; this gives the magnified angle 35', and dividing by the power 460, which he used, he obtained 4" 34"' for the distance of the centres of the two stars. The scale of the micrometer at this very convenient dis tance, with the power of 460 (which his telescope bears so well on the fixed stars that for near a twelvemonth past he hardly used any other) is above a quarter of an inch to a second ; and by putting on his power of 932, which in very fine evenings is extremely distinct, he ob tained a scale of more than half an inch to a second, without increasing the distance of the micrometer ; whereas the most perfect of his former micrometers, with the same instrument, had a scale of less than the 2000th part of an inch to a second.