933 



CIRCLE, ASTRONOMICAL. 



'CIRCLE, ASTRONOMICAL. 



934 



microscopes. For actual adjustment, the following directions may be 

 useful : A micrometer microscope is adjusted when the image of the 

 divisions of the circle is formed upon the cross wires of the micro- 

 scopes, and its size is exactly equal to five revolutions of the screw. 

 Suppose the vision of the wires and of the divisions to be perfect, but 

 that the space of 5' is passed over by four revolutions and fifty parts of 

 the screw. It is evident that the image is too small, and it must be 

 made larger by drawing out by the screw the object glass o, and bring- 

 ing it nearer the limb. In this case, however, the image is formed 

 between the wires and the eye-piece, and therefore the whole body of 

 the microscope must be drawn from the circle until the image falls on 

 the cross wires. It will now be found that the image of 5' is larger 

 than before, and it must be again measured by the screw. In this way, 

 by gradual attempts, the microscope may be perfectly adjusted, but the 

 operation, if complete accuracy be aimed at, is very fatiguing and 

 troublesome. 



The general form of the altitude and azimuth circle will be seen 

 from the woodcut. The lower, horizontal, or azimuth circle, is fixed 

 on three strong radii, which have foot-screws at their extremities, and 

 a strong vertical steel axis rising from their intersection. On this axis 

 the whole of the upper wofks move freely and smoothly on a conical cap 

 accurately fitted and ground to the top and bottom of the axis. On 

 the lower part of the cap is fixed a broad circular plate on which the 

 two pillars rest which support the circle and telescope. The micro- 

 scopes for reading the azimuth circle are fixed on the pillars. Out- 

 riggers stand on the top of pillars, each of wliich carries a notch or Y, 

 which can be raised or depressed by a screw. The vertical, altitude, 

 or declination circle is framed with a view to strength and lightness, 

 and its divided face is read by two microscopes placed on the end of the 

 curved tube, which is formed something like a pair of horns, and fixed 

 across the top of one of the pillars. The circles are usually divided to 

 5', and subdivided by the micrometer microscopes to seconds. The 

 graduation of the horizontal circle generally runs from to 360 ; the 

 vertical circle is differently divided, according to the pleasure of the 

 artist or observer. A little consideration will enable any one to apply 

 the rules given for reading microscopes to each particular case. We 

 shall speak indifferently of altitude and zenith distances, since they are 

 immediately derivable one from the other. The wires of the telescope 

 are illuminated by a lamp which shines in at one end of the cross or 

 transit axis of the vertical circle, and is reflected on the wires by a 

 pierced polished ring set diagonally. There are two sensible ground 

 levels, one of which is attached to the horns supporting the upper 

 microscopes ; another, which is free, can be applied through the open- 

 ings of the circle to the pivots of the cross axis. The cross level and 

 its adjusting screws will be understood by the accompanying figure. 



There are clamps and slow-moving screws to each circle to fix them, 

 and to give the power of bisecting any object with the telescope. 



Detached or Transit Level. 



The first adjustment is that of setting the vertical axis perpendicular 

 to Jthe horizon. Turn the instrument until the fixed level is nearly 

 parallel to two of the foot-screws, and place the bubble in the middle 

 by the foot-screws. Then, moving the instrument half round in azi- 

 muth, bring the bubble again to the middle, half by the foot-screw.s, 

 and the other half by the screws which raise or depress one end of the 

 level If this is carefully done, and the circle restored to its original 

 position, the bubble will be still in the middle ; but if not it must be 

 brought there, half by the foot-screws, and the rest by the level-screw. 

 Now turning the instrument a quarter round, through 90 .bring the 

 bubble into the middle of the scale by touching the third foot-screw. 

 Thig is generally placed in front and towards the observer, and rests on 

 a small lever apparatus, represented in the figure, which supplies a very 

 delicate elow motion, contrived, we believe, by Borda. When the 

 eration U successfully performed, the bubble will remain in the 

 centre, whatever azimuth the instrument is directed to. It is not 

 acewary in practice to touch the screw which adjusts the level if the 

 OTor be very email. The divisions of the level-scale corresponding to 



joth ends of the bubble should be noted before and after reversion, 

 and the mean taken. Let the foot-screws be called E and w, and the 

 ends of the level towards them be distinguished by the same letters, 

 and the scale of the level be numbered from the centre to each end, 

 ind suppose the readings be 



E. 



In the first position . 67 '4" 

 In the reversed position 59 '6" 



W. 



48-2" 

 56-0" 



Mean . . . . 63'5 52'1 difference = 11 -4". 



The mean readings would have been the readings of the level, if thu 

 .evel itself had been in adjustment. It is evident that E is too high, 

 or w too low, by half the difference, or by 5'7", and one of the foot- 

 screws must be turned enough to shift the bubble 5'7" in the right 

 direction. Troughton always moved both E and w an equal quantity 

 in contrary directions for this adjustment, which in many cases is 

 very much better. If that is carefully done, the E readings will bo 

 diminished, and the w readings increased by 5'7", and they will stand 

 thus : 



E. w. 



In the first position 

 In the reversed position 



61-7 

 53-9 



53-9 

 61-7 



or the bubble keeps the same position in its scale before and after 

 reversing, and the axis is vertical to the line joining the two foot- 

 screws. The bubble must be placed at the same divisions when over 

 the third foot-screw, and the axis is truly vertical. The difference 

 between the last-mentioned readings is 7'8", and the half of this, or 

 3'9", is the quantity through which the bubble should be moved by 

 the level-screw to complete its adjustment, if that be desired ; but if 

 the true value of the divisions of the scale be known, and it is also 

 known that this does not sensibly vary with the temperature or from 

 other causes, it is useless to adjust very scrupulously, since the ends of 

 the level must always be read and noted, and the difference allowed for 

 by a careful observer. In some larger instruments of this construction, 

 the vertical axis is adjusted by a plumb-line, which Ramsden and 

 Troughton applied in different ways. One of these applications will be 

 described under ZENITH SECTOK ; but we are inclined to think that the 

 ground-level is more accurate, while it is incomparably more convenient 

 than the plumb-line, for small circles, which move freely in azimuth, 

 and are used agreeably to their construction. The vertical axis being 

 properly placed, the cross axis of the upper circle is next to be set hori- 

 zontal. Pass the detached level carefully through the openings in thu 

 circle, place its notches on the pivots of the axis, note the ends of the 

 bubble, exactly as before, reverse the level, and note the ends again. 

 Suppose the results to be 



East End. West End. 

 First position . . 6'24 8'43 



Reversed position . . 7'96 671 



Mean . 



. 7-10 



7'57 



Hence the difference between the mean readings is 0'47, or the west 

 end is higher than the east by 0'235. The bubble must be moved this 

 quantity towards the east by raising the east or depressing the west 

 pivot. There is a screw under each pivot for that purpose. It is con- 

 venient to have the scale divided into parts, of which the units are 15" 

 of a degree. 



The microscopes should be adjusted, if that be required, in the 

 manner already described, but we recommend amateur observers, at 

 least, to have the body of the microscopes made unalterable, after the 

 maker has settled both the adjustments, by pinning together the body 

 and tube which carries the object-glass. Both adjustments will then 

 be performed by making the 5' spaces equal to five revolutions of the 

 micrometer screw, simply by shifting the body of the microscope a 

 little nearer to or farther from the instrument. This should be done 

 pretty carefully, but any remaining error may be corrected as in the 

 mural circle. The microscopes (there are generally a pair to each 

 circle) are finally to be placed in a diameter of their respective circles, 

 and nearly in the middle of the concentric lines, which contain the 

 divisions. There are screws in the tubes by which the microscopes are 

 mounted, which allow them a little motion in any direction, and the 

 cross wires are thus brought to bisect divisions 180 apart. This 

 requires no particular exactness, and may be done at once for the 

 azimuth circle. The upper microscopes may be adjusted approx- 

 imately. The optical part of the instrument must now be put into 

 order. At the eye end of the telescope there are generally five or 

 seven vertical and three or five horizontal wires, that is, fine lines % of 

 cobweb, raw silk, or any very delicate and uniform fibre. To place 

 these in the focus of the object-glass, apply the highest magnifying 

 power you have got, bring the wires to distinct vision, and look at a 

 bright star. The wires and eye-piece are fixed on a piece of tube 

 which moves stifHy within the tube of the telescope, and this must be 

 pushed in or drawn out till the star is seen perfectly round and sur- 

 rounded with rings, the wires being at the same time sharply defined. 

 This adjustment may be completed or verified thus : The pole star, or 

 any circumpolar star, may be bisected by the vertical wire when near 



