1" - 



ASTRONOMY. 



[EHROB or OOLLIHATIOV. 



M- 



and the probable error 



s. a, 



u>- J (0-649 + 0406 sec. 1 1). 



It would, thorofore, appear that his hearing had 

 improved, whilst his sight had remained the same 

 as before. This determination rests on 300 culmina- 



Robinson has recently investigated this subject, 

 and has introduced another error arising from atmo- 

 sphoric tremor, which he considers to vary as the secant 

 of tin- zenith distance. From his observations in 1830, 

 he deduces the following probable errors for a star wliose 

 declination ia i : 



-0-0445 y = 0-0619, Z, or the atnco spheric 



s. 

 tremor -. 0-0381. 



Several years afterwards, he found the following 

 values : 



. S. 8. 



x- 0-0732 y= 0-0554 Z = 0-049. 



But between the periods of the two observations, new 

 and more numerous wires had. been inserted in the tran- 

 sit instrument. 



Astronomers are very careful that the three adjust- 

 ments of the transit instrument, before mentioned, are 

 satisfied. And, firstly, with regard to the line of colli- 

 mation. This adjustment is generally made by the 

 observation of a well-defined terrestrial mark in one po- 

 sition of the axis, measuring its distance from the central 

 vertical wire (in terms of the micrometer) with which 

 the telescope is furnished. If, on reversal of the axis, 

 mark be again seen at the same distance from the 

 central wire, the collimation of the instrument is correct ; 

 but, if not, the wire frame must be moved to one-half 

 the difference of the two bisections. In the practice of 

 modern observatories, the complete correction of this 

 error is seldom attempted ; this adjustment, though 

 probably in large instruments the most permanent, being 

 nearly corrected, and then left for occasional verifica- 



The following extract from the Greenwich Obterea- 

 tiont, 1850 (Introduction, page xvi.), will show the 

 method of determining the error of collimation : 



" The value of this error, which is given in seconds of 

 space, is supposed positive when it implies an additive 

 correction to the transits of stars above the pole. For its 

 measurement the following method has been used : 



" A small transit instrument is fixed on temporary 

 wooden piers in the north opening of the transit room, 

 and its object-glass is turned towards the principal tran- 

 sit instrument. In this position, when the principal 

 transit instrument is turned towards the small one, the 

 wirer of the latter are seen as well-defined marks at an 

 infinite distance, and the error of collimation of the 

 principal transit may be determined by repeatedly re- 

 versing it, and viewing these wire*. 



" In November, 1846, a new determination of the 

 value of the micrometer-screw (of the transit telescope) 

 was made by means of six transits of Polaris over the 

 two wire* moved by the micrometer. The mean of the 

 intervals of the times of transit was found to be 

 I 3m. Is. -17 ; and the north polar distance of Polaris being 

 1 3O* 9" -3, this corresponds to an interval of space 

 188. But, by bringing each micrometer wire several 

 < into contact with the fixed central wire, this inter- 

 ! val was found to correspond also to 4r. -3G8 of the micro- 

 meter. Hence, one revolution 16* -297. 



" April, 29d. 22h. Th 1 81 II ^lena transit instrument 

 was used in the manner stated above. The wire ap- 

 peared as a broad white lino, there being no reflector ; 

 and the edge* of this lino were observed alternately by 

 placing upon them the micrometer wire of the transit 

 .... 



" OBSERVER M. 



Illuminated End Bait. 



8. 



Micrometer reading on coincidence with oolli- 

 mator (12 measures) 10-494 



Illuminated End West. 



Micrometer reading on coincidence with colli- 



mator (12 measures) . . . 8 '895 



Illuminated End East. 



Micrometer reading on coincidence with colli- 

 mator (12 measures) . . ... 10-500 



Illuminated End Wett. 



Micrometer reading on coincidence with colli- 



mator (12 measures) 8 873 



Hence reading for true line of collimation by 



1st and 2nd sots 9'695 



Hence reading for true line of collimation by 



3rd and 4th sets 9-687 



Beading for true line of collimation . . . 9 "691 



Micrometer reading on coincidence with D or 



central vertical wire . 9 -074 



Hence apparent error of collimation for D . 0-017 



" The illuminated end of the axis was left east, which 

 was also the position of the micrometer head ; and, as 

 the readings of the micrometer increase as the wire 

 moves from the head, D (or central vertical wire) was 

 therefore east of the line of collimation, and stars pass it 

 too late, or the error of collimation of D is Or. -017, or 

 0*-277. Also, with the illuminated end of axis east. 

 Polaris passes the mean of wires earlier than it passes D 

 by Os. -07, which in arc is equivalent to 0* -03 ; and the 

 correction for diurnal observation is 0""19. Hence 

 the corrected error of collimation is 0"-44." 



The second method of determining the error of colli- 

 mation is by observations of the transit of Polaris, or 

 any other close ciroumpolar star, in one position of the 

 axis, compared with its transit in a reversed position of 

 the axis. By proper apparatus, this reversal of the in- 

 strument can be effected during the time of transit. If, 

 therefore, three wires of Polaris, or X Urste Minoris, be 

 observed in one position of the axis, and the remainder 

 of the transit in a reversed position, and if each set of 

 wires be separately reduced to the central wire, half the 

 difference of the two results will be the error of collima- 

 tion for the object observed, which can be reduced to the 

 error of collimation (in arc) for an equatorial star 

 (dividing by the proper factor). It is necessary, how- 

 ever, in the use of this method, that the reversal be very 

 carefully made, and that the mean of several results of 

 separate stars be taken. 



The following example of this method is extracted from 

 the Georgetown Observation* : 



Transit of X Urste Minoris on August 7th, 1845. 

 End C of the axis to the east. 



li. m. a. 



At 3rd wire . 20 4 24 

 Movable wire z 20 11 3 



4th wire . 20 17 48 Here the Imtrument WM iwawd. 



x 20 24 31 

 3rd wire . 20 31 10 



Interval from 3rd to 4th wire 

 4th to 3rd wire 



m. s. 



.13 24 - t 

 .13 22 - t o 



Difference 



2- tto 

 t-to 



1 - 



