134 



KNOWLEDGE 



[June 1, 1896. 



PHOTOGRAPH OF THE CLUSTER MESSIER 

 24 CLYPEI. 



By Isaac Kohkuts, D.Sc, F.R.S. 

 R.A. 18h. 12m., Decl. S. 18' 28'. 



Til]'', photograph covers the region bctwceu R.A. 

 18h. !im. l.'is. and Ish. Kim. 23. Declination 

 between I?'' 49' and 19'' 2' south. 

 Scale, one millimetre to twenty-four seconds of 

 arc. 



Co-ordinates, for the epoch a.d. 1900, of the fiducial 

 stars marked with dots. 



Star (•). D.M. No. 1801. Zone - 18''. R.A., 

 I8h. 9m. 883. Decl., S. 18'=' ■11-4'. Mag., 6-8, 



Star (■•). T).M. No. 4880. Zone - 18'^. R.A., 

 18h. 11m. :!7'ls. Decl., S. 18° 80'()'. Mag., G-7. 



Star (.•.). D.M. No. 4896. Zone - 18°. R.A., 

 18h. 12m. 51s. Decl., S. 18' 89-6'. Mag., 73. 



Star (::). D.M. No. 4900. Zone - 18°. R.A., 

 18h. 12m. 5(i-4s. Decl., S. 18' 12-5'. Mag., 8-2. 



Star (:■:). D.M. No. 492G. Zone - 18°. R.A., 

 I8h. l.'im. 28'7s. Decl., S. 18° .54'2'. Mag., (iO. 



The photograph was taken with tlic •20-inch reflector on 

 August 14th, 1895, between sidereal time 18h. 57m. and 

 20h. 57m., with an exposure of the plate during two hours. 

 References. 



The cluster is N. G. C, No. 6G03 ; G. C, No. 4397 ; 

 and h 2004. 



Sir J. Herschel (G. C, 4397) describes the cluster as 

 remarkable, very rich, very much compressed, round, and 

 the stars of the 15th magnitude. 



The photograph shows the cluster to be in shape some- 

 what resembling a horseshoe, with the open side pointing 

 towards the narth prccaUmi, and the stars, both in it and 

 in the regions around, ar-e in lines and curves of various 

 form. The stream of the Milky Way is well shown on 

 the photograph, with dark spaces in which are few stars, 

 and in some places none ; the brightest star on the plate 

 is only G'8, and the faintest about 17th magnitude. 

 The white line which is shown on the cluster consists of 

 five or six stars so close together that their photo-images 

 overlap ; and on several other parts of the plate where 

 the stars show an elongation, it is due to double or multiple 

 stars. 



There are doubtless many of our readers who possess a 

 telescope which is eqnatorially mounted, and would like to 

 examine visually those parts of the sky where are the mar- 

 vellous groups and curves of stars which are shown on the 

 annexed as well as on other photographs already published 

 in Knowledge ; and, in order to enable them to point their 

 telescopes upon such objects, the application of the following 

 simple methods will suffice. 



Let us suppose that it is desired to determine the co- 

 ordinates (right ascension and declination) of the centre 

 of the small group of stars sixty-six millimetres from the 

 north edge and eighty-five millimetres from the priredimi 

 edge of the photograph annexed ; and also to ascertain 

 what would be the diameter of a circle drawn on the 

 photograph that would be equal to the field of view of an 

 observing telescope, which we will assume to be five 

 hundred and fifty-two seconds of arc in diameter ? 



Two questions are herein involved, and we will answer 

 the second in the first order. The scale of the photograph 

 is, as ah-eady stated, twenty-four seconds of arc to one 

 millimetre, and the field of view of the observing telescope 

 five hundred and fifty-two seconds of arc ; the measured 

 diameter, in millimetres, of a circle drawn on the photo- 

 graph will therefore, obviously, be ■''/^?" = 23 millimetres. 

 If we take, between the points of a pair of compasses, the 



radius 11*5, and with it describe a circle on a piece of 

 paper; then cut out the interior and apply the vacant 

 circle of twenty-three millimetres in diameter to any part 

 of the photograph, it will, approximately, represent the 

 telescopic field of live hundred and fifty-two seconds of arc 

 in diameter ; where, within a corresponding coincident 

 ai-ea in the sky (if the telescope is of sufficient power to 

 show stars down to about the 17th magnitude), all the 

 stars shown on the photograph would be visible. 



The other question— how to determine the co-ordinates 

 of the positions of any star or other ol)ject shown on the 

 photograph — will be answered in the following manner : — 



First. — How to find the right ascension of the centre of 

 the group of stars referred to above : — 



TIm' R.A. of fill- (icluciiil star ( : :) - 181i. 12m. .56-48. 

 „ (•■ ) ^ l«'i- l'"i 37-18. 



Oh. Im. 19-3s. = 79-3s. 

 (the difterence in R.A. between them). We must now 

 convert these 79-3 seconds of time into millimetres for 

 measurement purposes, and that can readily be done by 

 aid of the table annexed, which is computed to show the 

 value of one millimetre, in seconds of time, at each degree 

 in declination between the equator and the poles. The 

 declination of fiducial star (: :) is 18' 12-5' sauth .- and by 

 referring to the table we find the value of one millimetre 

 at eighteen degrees from the equator to be 1-68 seconds of 

 time in R.A., and therefore ^y^J';,,, = 47'2 millimetres. 

 Take this distance between the points of a pair of dividers ; 

 place one point on the centre of the fiducial star (: :) 

 and extend the other point towards the /nrreilini/ side ; 

 at the same time place a rule, having a fine edge, in north 

 and sy((f/i direction so as to bisect both fiducial star (•• ) 

 and the disengaged point of the dividers. 



The edge of the rule is then on the meridian 

 (18h. 11m. 37-ls.) of the fiducial star ( • • ) ; and whilst 

 the rule is firmly held in this position, measure the 

 distance of the centre of the group of stars herein referred 

 to from it, which will be found to be 3'2 millimetres. 

 Then 3-2 x 1-G8 ^^ 5-37 secorids of time, being the 

 distance in R.A. of the centre of the group from the 

 meridian of fiducial star ( ■ • ), and 18h. 11m. 37-ls. -f 

 5-37s. = 18h. 11m. 42-47s. is the required R.A. of the 

 centre of the group. 



Second. — To find the declination of the centre of the 

 group : — 



The declination of the iiducial star ( - • ) is 18" 30 0' . 



(::) „ 18° 12-5' 



0° 17-5' = 1050" of are, 



which is the difference in declination between the two 

 stars ; and since the scale of the photograph is twenty- 

 fourseconds to one millimetre, 1050' : 21" — 43-75 milli- 

 metres. Take this distance between the points of the 

 dividers, and place one point on the centre of fiducial 

 star ( • • ) and extend the other point towards the north .- 

 at the same time place the rule with its edge in the 

 direction of a parallel of declination, and bisecting both 

 fiducial star (: : ) and the disengaged point of the dividers. 



The edge of the rule is then in the parallel of declination 

 of fiducial star ( : : ) ; and whilst the rule V3 firmly held in 

 this position, measure the distance of the centre of the 

 group of stars from it, which will be found to be 6-5 milli- 

 metres. Then ()-5 x 24" = 2-G minutes of arc, and by 

 adding this quantity to the declination of iiducial star ( : : ) 

 we obtain the declination required of the centre of the 

 group : which is 18° 12-5' + 2-G' = 18^ 15-1'. Refraction 

 in this case will be a small quantity, but if required the 

 correction for it can be made in the usual way. - 



The fiducial stars ( • • ) and ( : : ) have been chosen in 



