398 Colours of Cor Caroli. 



COLOURS OF COR CAROLI. 



On this subject we have received the following interesting 

 letter from Mr. George Knott. 



May 18th, 1865. 



Deak Sir, — In accordance with your invitation in the May number 

 of the Intellectual Observer, I have much pleasure in sending you 

 the following estimate of the colours of the components of the 

 double star, Cor Caroli. 



1865, May 16. Instrument employed, a 7^ inch Alvan Clark 

 Refractor. Thornthwaite's Aplanatic, powers 102, 173. 



A. Very pale yellow, almost white. 



B. Pale lilac. 



Using Admiral Smyth's chromatic scale the colours would be 



A. Yellow*. 



B. Reddish purple*. 



I find in my Journal the following rather interesting observation 

 of the colour of B of Cor Caroli by daylight. I quote from my 

 Journal: — 



" 1862, Nov. 27, lOh. 15m. a.m. Had a fine view of the double 

 star, Cor Caroli. The small star was in colour a little more dusky 

 than the deep blue of the sky. I should call it deep dusky blue or 

 lilac. 7^ inch achromatic." 



I think that it would be convenient in practice, particularly 

 where space is precious (as, for instance, on a form for the entry of 

 double star measures), to use a large letter to express the dish colour 

 from the scale, and a small letter for the qualifying adjective. Thus, 

 in the case of Cor Caroli, instead of writing the words, " yellow 4 ," 

 "reddish-purple 4 ,'' we might write " Y 4 ," "rPV And in the same 

 way the colour estimates of other stars might be recorded, as in the 

 case of the following stars observed by me also on the 16th inst. : — 



a Herculis A = O 3 , B == bG 4 . 

 95 Herculis A = G 4 , B = O 3 . 

 j8 Cygni A = Y 3 , B = bG 4 . 



There may be cases, as you justly remark, in which a mere quali- 

 fying adjective will hardly suffice as indicative of tint; these it will 

 not be easy to deal with. Any one with an artistic taste, and 

 accustomed to mix colours, might perhaps indicate a tint by a combi- 

 nation of several disk colours. It is probable, however, that his 

 combination would convey but a very indistinct idea of hue to the 

 uninitiated. 



Thornthwaite's aplanatics are very pleasant eye-pieces to use in 

 colour estimations ; unfortunately they labour under the disadvan- 

 tage of a rather annoying ghost. It is necessary, too, to pay some 

 little attention to the proper cleansing of the field lens, and in those 

 I have the field is not perfect to the edge ; there is a band of distortion 

 which one would be glad to get rid of. In the " Monthly Notices, 



