396 PEOFESSOR C. PIAZZI SMYTH ON THE 



indeed, heard of a colour-blind friend — of the orthodox order, which cannot per- 

 ceive any difference between the brightest red and the most brilliant green— say 

 of another who mistook between some shades of blue and green, " how extraordi- 

 nary ! what a stupid fellow* he must be ; now, if he had mistaken red and green 

 there might have been some excuse for him." Yet the sentiment of the world at 

 large I conclude will be, notwithstanding, that certain varieties of blue and green 

 are very easily mistaken for each other when viewed under varying contrasts 

 with other colours. 



In Position and Distance, these stars are a caution ; and in Character, who 

 shall say what they are, though, too, they have been so abundantly observed. 



" Fixity," relative, from the earliest time, has been strongly claimed for them 

 by some good judges; but in that case, what becomes of the reputed accuracy of 

 the great double star-measurers ; and what is the advantage of their occasionally 

 publishing their position angles to the i^oth of a minute, and the distances to 

 the roioo of a second, — when a discrepance of no less than five whole degrees in 

 position, and half a second in distance, is shown, by thus bringing together 

 various records of a Herculis, to be a very common error for a long and well 

 weighted series of observations. What, also, at this rate, is likely to be the value 

 of a double-star orbit, computed on a small number of observations extending over 

 no more than a very moderate portion of its periodic time ? 



Granting, however, for argument, the imputed relative fixity, these stars 

 would become, under the B, A. C. proper motion test, " slow binaries;" seeing 

 that their "position" should otherwise have increased twenty-one degrees in the 

 last thirty years. 



But then, again, other authorities have challenged the truth of the B. A. C. 

 proper motion in this case, which they considered extremely erroneous ; and the 

 late Captain Jacob, when at Madras, undertook a series of most carefully 

 conducted equatorial observations, to ascertain "s^hether the group was, in cha- 

 racter, optical or physical. These observations have been fully printed by the 

 Royal Astronomical Society, and resulted in shoAving with remarkable force, in 

 as far as the agreement of the observations, inter se, could make it, that the 

 group was not only optical, but that the annual parallax of one member upon the 

 other was sensible, and even well determined in its amount from the measures. 

 Yet its full effect, only reached, between its two six-monthly extremes, to one- 

 fifth of the difference between his mean of measures and that of Dr Luther taken 

 at nearly the same epoch ; while Dr A. Auwers, on the mean of four observations 

 on each night, has in the year 1861 a difference, reaching, in the short space of four 

 nights, to three times the whole extent of Captain Jacob's imputed annual 

 parallactic effect. 



* It was not M. Struve, nor indeed any astronomer whatever, who was thus referred to. 



