Ml 



CIRCLE, ASTRONOMICAL. 



CIRCLE, ASTRONOMICAL. 



920 



however, another and more sensible advantage in two opposite readings 

 above one. Conceive the whole circle to be moved towards z, or z', or 

 pushed sideways towards N or s ; still, so long as E o continues to be 

 parallel to itself, which it must be if it continues to be in the direction 

 of a very distant object, the sum of the readings of A and B will 

 remain the same ; whatever one loses the other will gain. Hence it is 

 not necessary that the axis should be truly circular, or that the centre 

 of the axis should be exactly concentric with the centre of the divisions, 

 which last requisite is not easily accomplished in the present con- 

 struction of English dividing engines. In all circles which have read- 

 ings diametrically opposite to each other, the observer ought to consider 

 each pair as only one reading of a diameter, and not as two unconnected 

 readings. There is another form in which a circular instrument for 

 measuring altitudes may be constructed ; the divided circle may be 

 fixed to the block, and the line of sight, E o, turn upon the axis c. In 

 this case the pointers must be connected with E o, and revolve on the 

 same centre. All the previous remarks are equally applicable. 



The operation of noting the angles, or of reading off, has been 

 described in its rudest form, as it seems to have been practised by the 

 Greeks. We must here explain somewhat more minutely the use of 

 the micrometer microscope. (For other modes of subdivision, see 

 VKBXIEB.) 



Fig. 2. 



Fig. 3 



Fia 2 is a micrometer microscope : A B is supposed to be the space 

 between two divisions of the circle which is generally equal to 5 . 1 he 

 object glass c, which is sometimes, and with great benefit, made achro- 

 matic, forms at a b an inverted image of A B, as a 6. Within the body 

 of the microscope there is a slide, represented in ^ 3. This slide is 

 drawn forwards by a fine screw (generally of 100 threads to an inch), 

 while it is held back by a spring to avoid lost time; it has two fine 

 "s crossing at an acute angle, as in the figure. The moage A B should 

 1 exactly on this cross, and the two lens eye-piece should be so placed 

 to rive distinct vision of both. Also five revolutions of the screw 

 "ould move the cross exactly from any division of the circle to the 

 ne Khbouring one. The circular plate H is fixed by friction upon the 

 1 and is divided into 60 parts, which correspond to seconds on the 

 oirde and are read off by a pointer, I. The entire revolutions of the 

 are counted by a sort of comb, or indented plate fixed near 

 r but not moveable with them. There ,s a little projecting 

 of metal opposite the cross wires, which is seen between the 

 cof thTcomb at each whole revolution of the screw. This micro- 

 ?ne having it* parts at the proper distance from each other and 

 teed at the proper distance from the divided circle must be 

 "onue 



p ) tob S usttuorep. 



,omb, and the zero of the divided head opposite to its index. 

 v i,m of the circle seems exactly to cut the cross at the angular 

 e observer has only to note what division of the circle it is, 

 'nl ttotisthV^ading, just as with the simple pointer. Generally, 



: one dS" U "PP 6 " ab Ve and an0thCT belo , w ^, e C T' 

 cHhe divisions are seen inverted, the division apparently below the 



i, the one really above, which, according to fy. l,and the position 

 is the division immediately less than the position of the microscope 

 winter Turn the screw round until the appearance is exactly the 

 fa fig 4, and suppose that the tongue has moved over two 

 fTth Showing two revolutions of the screw, and the index is half way 

 2rf 34 on the divided head, then 2' and 33'5" are to be 

 " th, degree* and minutes of that division. Now that division 



is 50 6' ; therefore the true reading is 50 7' 33'5". What has been 

 said as to error of collimation holds good with a microscope as with a 

 pointer. When the line of sight E o falls between z and N, then the 

 next least division will be really below, but apparently above the cross, 

 and the measurement should be made to it. The screw in this case 

 must be turned the other way, and instead of the divisions of the head 

 shown by the index, their difference from 60 must be taken for the 

 seconds. A little caution and experience will guard the observer 

 against mistake. The length of the microscope can be altered by 

 screwing the tube in which o is fixed, and the microscope can be shifted 

 in its support to and from the circle by the nuts K K, k i: It is evident 

 that by these two movements the size of the image can be altered as 

 well as its situation with respect to the cross wires. (See the figure of 

 the altitude and azimuth circle for the mode of mounting the micro- 

 meter.) The apparatus of the micrometer microscope, shown at fy. 3, 

 is frequently attached to the eye-end of a telescope, and is placed in 

 the focus of the object glass. The wires are then placed across the 

 slide and at right angles to it, so as to be parallel with other wires of 

 the telescope. If these points are clearly understood, the reader will 

 find no difficulty in understanding the rest of this article. [MICRO- 

 METER.] 



The earliest application of a circle to astronomical purposes is 

 described by Ptolemy (' Almag.,' i. 10), who calls it, from its use, a 

 solsticial circle. This consists of a smaller circle turning freely within, 

 and in the same plane with, a larger and fixed divided vertical circle. 

 Two small projecting prisms are placed in a diameter of the inner 

 circle for a line of sight, and pointers are also fixed on the inner circle, 

 which move on the face of the outer circle. The instrument thus 

 formed is to be carefully adjusted by a plumb-line and meridian-line in 

 the plane of the meridian. In observing the sun, the inner moveable 

 circle was turned round until the shadow of the upper prism exactly 

 covered the lover prism, when the pointers marked the corresponding 

 division upon the outer fixed circle. The inner circle seems to have 

 been employed as the most accurate mode of giving a rotatory motion 

 to the lint of collimation, concentric with the divided circle. If a bar, 

 carrying the line of sight and the pointers, be supposed to revolve on 

 an axis, it is the second case oijig. 1. The language of Ptolemy does 

 not inform us who was the inventor of this instrument, or even that 

 it was ever made or used. Delambre conjectures, with some proba- 

 bility, that it is due to Eratosthenes, though he inclines to think that 

 the celebrated measure of the obliquity of the ecliptic by that astro- 

 nomer namely, that the distance between the tropics is equal to |J of 

 the circumference was deduced from observations with the gnomon. 

 However that may be, it is certain that'the solsticial circle was the best- 

 contrived instrument of which we find any account until the tune of 

 Roemer ; and it bears, as will be seen, a very close analogy to the 

 modern mural circle of Troughton. 



Except the complicated astrolabe of Hipparchus, which consisted of 

 five concentric circles, so contrived as to have one circle in the plane of 

 the ecliptic, and another at right angles to it, we do not find the entire 

 circle employed in large instruments before the time of Tycho Brahe', 

 and then only for an equatorial. The supposed authority and example 

 of Ptolemy, who proposed a quadrant, and the desire of increasing the 

 sensibility of instruments, by enlarging the scale of the divisions, 

 blinded astronomers to the more solid advantages of the entire circle. 



In 1704 Roemer erected his Rota meridiana, or transit circle, in his 

 private observatory l m due west of the astronomical tower of Copen- 

 hagen. He had invented the simple transit and the altitude and 

 azimuth circle about fourteen years earlier, and in this new instrument 

 he very (happily combined the qualities of a transit telescope with 

 those of a meridian altitude circle. In a letter to Leibnitz, 15th 

 December, 1700, he had already said, "that the quadrant and sextant 

 ought no longer to be used, and that he would rely more on an entire 

 circle of four feet than on a quadrant of a ten-feet circle." (' Miscell. 

 Berolin. continuat.' ii. p. 276.) A description and figure of the Rota 

 meridiana are to be found in the ' Basis Astronomis;,' p. 143, tab. viii., 

 of his pupil Horrebow, ' Hafnise,' 1735, with a specimen of three days' 

 observations. The altitude and azimuth circle is described in p. 4;. 

 and figured in tab. ii. of the same work. 



The Rota meridiana of Roemer was a divided circle set upon a 

 hollow double cone, at right angles to the axis of the cones, and 

 concentric with them, the circle being near one end of the axis. The 

 divisions were read by two microscope verniers, fixed in a diameter 

 of the circle on one of the supports of the axis. For general pur- 

 poses, this is perhaps as well a designed instrument as has ever been 

 constructed, yet it was not imitated till a similar construction was 

 recommended by Mr. Pond, and adopted for the observatory at 

 Paramatta. A second, on a much larger scale, was somewhat later 

 made for the observatory at Oxford ; both these were by Mr. T. Jones. 

 Notwithstanding the advice and practice of Roemer, it does not appear 

 that the entire circle was used for astronomical purposes until Mayer 

 proposed his repeating reflecting circle. His description was published 

 in 1770 (' Tabulsc Solares,' p. 21, plate ii.), but the repeating reflecting 

 circle did not come into use until modified and perfected by Borda 

 about 1787. 



In 1784 the Danish astronomer Bugge published his ' Observations 

 Astronomicso, Haunia;,' in which he gives a figure and description of 

 a vertical circle of four feet diameter revolving in azimuth, p. Hii. 



