212 



MICROMETER. 



plane of the circular board, where a double-jointed 



handle e P commands its motion. By this contrivance 

 we see t [, e arm L ma y be ijf te j up to any a i t j tu d e f rom 



Lucid, 

 Diic, and 

 imous 



crometers." * ne horizontal position to the perpendicular, or be suf- 

 x Y-""' fered to descend by its own weight below the horizon- 

 tal to the reverse perpendicular situation. The weight 

 of the handle P is sufficient to keep the arm in any 

 given position ; but if the motion should be too easy, 

 a friction spring applied to the barrel will moderate it 

 at pleasure. 



In front of the arm L a small slider, about three 

 inches long, is moveable in a rabbet from the end L to- 

 wards the centre, backward and forward. A string is 

 fastened to the left side of the little slider, and goes to- 

 wards L, where it passes round a pulley at m, and re- 

 turns under the arm from m n, towards the centre, 

 where it is led in a groove on the edge of the arm, 

 which is of a circular form, upward to a barrel (raised 

 above the plane of the circular board) at r, to which 

 the handle r D is fastened. A second string is fasten- 

 ed to the slider, at the right side, and goes towards the 

 centre, where it passes over a pulley , and the weight 

 n, which is suspended by the end of the string, returns 

 the slider towards the centre, when a contrary turn of 

 the handle permits it to act. 



Two small lamps, 2 inches high, 1 i in breadth, by 1 

 in depth, are shewn at a and b. The sides, back, am 

 top, are made so as to permit no light to be seen, and the 

 front consists of a thin brass sliding door. The flame 

 in the lamp a is placed -^ths of an inch from the left 

 side, i%ths from the front, and half an inch from the 

 bottom. In the lamp b it is placed at the same height 

 and distance measuring from the right side. The 

 wick of the flame consists only of a single very thin 

 lamp-cotton thread ; for the smallest flame being suffi- 

 cient, it is easier to keep it burning in so confined a 

 place. In the top of each lamp must be a little slit, 

 lengthways, and also a small opening in one side near 

 the upper part, to permit air enough to circulate to 

 feed the flame. To prevent every reflection of light, 

 the side opening of the lamp a should be to the right, 

 and that of the lamp b to the left. In the sliding door 

 of each lamp is made a small hole with the point of a 

 very fine needle, just opposite the place where the 

 wicks are burning, so that when the sliders are shut 

 down, and every thing dark, nothing shall be seen 

 but two fine lucid points of the size of two stars of the 

 third or fourth magnitude. The lamp a Is placed so, 

 that its lucid point may be in the centre of the circular 

 board where it remains fixed. The lamp b is hung to 

 the little slider which moves in the rabbet of the arm, 

 so that its lucid point, in a horizontal position of the 

 arm, may be on a level with the lucid point in the 

 centre. The moveable lamp is suspended on a piece 

 of brass fastened to the slider by a pin exactly behind 

 the flame on which it moves as a pivot. The lamp is 

 balanced at the bottom by a leaden weight, so as al- 

 ways to remain upright, when the arm is either lifted 

 above, or depressed below the horizontal position. The 

 double-jointed handles r D, e P, consist of light deal 

 rods, 10 feet long, and the lowest of them may have 

 divisions, marked on it near the end P, expressing ex- 

 actly the distance from the central lucid point, in feet, 

 inches, and tenths. 



From this construction we see, that a person at a 

 distance of 10 feet may govern the two lucid points, so 

 as to bring them into any required position south or 

 north, preceding or following, from to 90", by using 

 the handle P, and also to any distance from /^ths of 

 an inch to 5 or 6 and 20 inches, by means of the 



handle D. If any reflection or appearance of light Lucid, 

 should be left from the top or sides of the lamps, a Disc, and 

 temporary screen, consisting of a long piece of paste- L "nfou" 

 board, or a wire frame covered with black cloth, of the " 5 <:terf 

 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, situated so that the 

 moveable lucid point may pass along the opening left 

 for that purpose. 



Fig. 1 5. represents part of the arm L, of a larger size ; Fig. 1 5. 

 s the slider ; m the pulley, over which the cord x t 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 F'g- '<>> IT; 

 lamps a, b, with the sliding doors open, to shew the si- 

 tuation of the wicks, tu is the leaden weight, with a 

 hole d in it, through which the wire R of Fig. 15. is to be 

 passed, when the lamp is to be fastened to the slider s. 

 Fig. 18. represents the lamp a with the sliding door Fig. 18. 

 shut ; I the lucid point ; i k the openings at the top, and 

 s at the sides for the admission of air. 



The ingenious artist will soon perceive that the 

 motions of this micrometer are capable of great im- 

 provement 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 obser- 

 ver goes to measure their distance, it will not be ne- 

 cessary to say more on the subject. 



Sir William Herschel next proceeds to show the ap- 

 plication of this instrument. It is well known to op- 

 ticians, 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 see a scale on which 

 the magnified picture is thrown. In this manner Sir 

 W. Herschel generally determined the power of his te- 

 lescopes ; 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 suffi- 

 cient 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 obstruc- 

 tion to the view : therefore, in using this instrument, it 

 is put at 10 feet distance from the left eye, in a line 

 perpendicular 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 lu- 

 cid points to a similar situation with the two stars; 

 and, by the handle D, the moveable lucid point is ap. 



