3S6 



KNOWLEDGE. 



October, 1910. 



INNES'S CLASSIFICATION. 



In the " Introduction " to his Reference ditiilof^iie 

 Innes broaches several topics of interest in connection 

 with Double Stars which have not hitherto been 

 ver\- fulh" or generallv discussed, albeit many of them 

 of real practical importance. He starts with a 

 question wliich, though it is a very obvious one. has 

 not, I t'unk, hitherto received much notice. '" What 

 is a Double Star ? '" Then he goes on to say : — 

 " Should the wide pairs of the earlier astronomers 

 be indiscriminately included ? It is easier to sa\' 

 ' No ' to the latter suggestion than to gi\-e a definition 

 of what should constitute a Double Star that will 

 meet all \-ie\\ s. A compromise has been adopted : 

 in addition to man\' rather wide pairs, worth\' of 

 inclusion from some point of view, ever\' known 

 southern Double Star finds a place here which 

 falls within the limits of distance gi\'en in the 



follow ing table : — 



M.iiliiitude of 

 Primary. 



1 



7 



Limiting distance of 

 Components. 



30" 



7~ 



3 



i 20 



4 15 



5 10 



6 7 



7 5 



8 i 



y 1 



"These limits do not exclude any Binary pair in 

 either hemisphere. Some systems, much exceeding 

 the above distances and showing no certain signs of 

 orbital motion, have been included on account of the 

 large proper motion common to both components." 



It must be understood that I am tiuoting the 

 foregoing as laving dow n a scheme for the compila- 

 tion of a Catalogue o{ Double Stars which is to be 

 based on certain definite principles. 



THE XOMENCLATCRK AND 11 )E.\TIF ICATK )X 



OF STARS. 



On the question of nomenclature Innes has 

 propounded some ideas eminently proper and reason- 

 able for the purposes of a publication issued under 

 official authorit\% and which ideas are especialh" 

 marked bv fairness towards indi\idual discoverers of 

 Double Stars. 



Innes has also discussed a matter of some practical 

 importance but of considerable practical difficult)'. 

 The Double Star observer in specif\'ing the magni- 

 tudes of the two stars which he has seen will sav 

 correcth' (or it ma\' ver\' likeh' be, incorrectK) that 

 the\- are of magnitudes so and so. The photometric 

 observer and the astronomical map-maker or editor 

 will for his purposes ignore the duplicit\- of the star, 

 and will record the two stars as one. giving the 

 conjoint magnitude. The problem will be — " \Miat 

 magnitude is to be assigned to the two stars 

 treated jointh as one?" Innes has suggested 

 a formula, and has framed a table by means 

 of the formula, which, though it is somewhat 

 technical and intricate, must receive some notice, 

 however brief, in this paper. The point iinolved 

 will be best understood b\' citing two ccmcrete 

 examples which he gives. 



(1) The magnitude of the very bright star a 

 Centauri is put as — 0'2. " \Miat are the indi\idual 

 magnitudes of A and B ? " He suggests that as the 

 difference between the two magnitudes is \'5, there- 

 fore the magnitude of A is — 0'4, and of 15 1"9, these 



figures lumped together photometrically combining 

 to make the naked eye magnitude of the Star, 

 regarded as one, to be 0"2. 



(2) The inverse case may be represented, sa}', by 

 taking two stars of equal magnitude, that magnitude 

 being t'.^i,', 6"8. The question is "What should be the 

 ajiparent magnitude of those two stars combined as 

 one ? " Innes answers the question by saving that 

 the naked-eye magnitude would be 6"0. 



Respecting his formula and table he confesses that 

 though the table {which depends upon the accej)ted 

 light-ratio of 2"512 to which the Diirchmusteiuiiii 

 magnitudes are supposed to conform), holds good for 

 most pairs, he is convinced that it does not apply to 

 pairs where both components are faint- Partly 

 because of this reser\-ation, and partly because the 

 subject invoh-es minuteness of detail with which I 

 fancv that few amateurs will care to trouble them- 

 selves, I do not propose to go further into the 

 matter, and shall content myself by saying that 

 when an observer desires to observe a pair of stars 

 presented to him as a Double (or Triple) Star, he 

 must add something to the specified magnitude of 

 Star .\ w hen he goes to the telescope to find the pair. 

 How much he should add will obviously differ in 

 ever\- case : but as a sort of rule of thumb addition 

 he will in most cases, I think, be safe in looking for 

 a star brighter by \ or }, a magnitude than the 

 printed magnitude affixed to Star .\. 



/ To be euntiuued.l 



