GREAT REFRACTING TELESCOPE AT ELCHIES, IN MORAYSHIRE. 399 



both stars equally, and the mean for each of them Avas little else, or did not 

 amount to anything more decided, than " greyish-yellow.'' 



Such then was the equality of nameless colour which I felt bound to assign 

 to the stars on two nights, though I was comparing them with the Cycle, and 

 the Cycle only, and the author of that standard book describes A, as being of 

 a " Light apple-green" colour, and B, " cherry red." On informing my Father 

 of these results, he called together a number of his observing friends in the follow- 

 ing summer, and, as detailed in the " Speculum Hartwellianum," p. 317, they 

 again made one star " green" and the other " red." 



My Teneriflfe observations therefore seemed utterly wrong, and without any 

 precedent, except that Sestini's observations in 1844-5, and given at p. 313 of 

 the same work, made them both " gold yellow." But might he be colour-blind, 

 seeing that the reputed tints are precisely those which are confounded by persons 

 with that peculiarity of vision ? Or again, seeing that both he, and myself on 

 Guajara, having given nothing but the colours, and not having identified the star- 

 group beyond all question by making micrometrical observations at the same 

 time of " Position" and " Distance,"— might we both have got hold of a wrong 

 star-group ? I could not say anything formerly to these questions, except that 

 no such suspicion crossed my mind on either of the two nights at Teneriffe, and 

 when more diflBcult stars still, were found ; but a new view of the whole case 

 has opened up, on referring, to Struve's original and individual observations in 

 the " Mensurse Micrometricse." 



In his mean result for the pair, he agrees nearly with all the rest of the world, 

 by calling one star " greenish-yellow," and the other " reddish-yellow ;" but 

 appended to his single observations we find as follows, — 



In 1828'7l A is Greenish-yellow, and B Reddish-yellow. 



In 182876 A is Greenish, and B Yellowish. 



In 1829-62 A is Yellow-green and B Egregiously red, 



with the note that the colours are " wonderful." 



I But in 1832-53 A is simply described as " yellower" than B ; from which it would 

 seem that they were then both yellow ; but one, a more pronounced hue of that 

 colour than the other. 



Now, on each of these four occasions, Struve measured both " Position" and 

 " Distance," thereby proving the identity of the star-group. He also, three times 



I over, demonstrates himself capable of distinguishing, or being affected by, green 

 and red colours, even to an extreme degree ; and yet this same, and most able 



i man, with the same powerful telescope on the fourth and last occasion, sees these 



distinguishing colours gone, and the two stars reduced to an almost equality of 



' colour. A case so observed cannot be slighted, and it forms the third occasion 



(the first in point of date) on which such a phenomenon has been noticed with 



95 Herculis ; let us inquire what interval of time separates the manifestations : — 



I VOL. XXIII. PART 11. 5 Q 



