442 Colours of Stars. 



fact which may be easily illustrated by observing the blue 

 aspect of the moon in the presence of a powerful lamp or gas- 

 light, what certainty can be obtained as to the real colour of 

 the smaller components of double stars, where the principal 

 has any decided hue? In many instances, as when yellow 

 stars are attended by little lilac comites, the suspicion of mere 

 contrast naturally obtrudes itself. In others where the tints 

 are not complementary, that of the smaller star is sure to be 

 modified in some way, unless the principal is white ; and even 

 then it is not impossible that a small white attendant, lying 

 within the outstanding blue fringe of the large star, might 

 receive an orange or tawny tinge from its position — an illusion 

 which I think I have noticed. The difficulty can only be fully 

 met by inserting a bar or thick wire in the field, and keeping the 

 larger star behind it by hand or clock motion, till the eye has 

 recovered from the impression of the stronger light. By this 

 method of artificial occupation Arago satisfied himself that most 

 frequently the colour of the smaller star was not the mere eifect 

 of contrast ; and Struve I. found that the beautiful blue of the 

 two companions o 2 Cygni (No. 58 of our Double Star List, In- 

 tellectual Observer, Nov. 1862, p. 304) was independent of 

 the presence of the large orange star. Where great accuracy 

 is desired, and a driving motion can be applied, it would be 

 advisable, after hiding the principal star, to close or avert the 

 eye for a short time, that an entirely fresh impression of the 

 colour of the companion may be obtained. 



When both eyes are of equal goodness, which is by no 

 means always the case, the employment of each in succession 

 in the examination of colour may prove an useful check upon 

 any accidental bias in either. A change, too, of eye-pieces 

 may always be expedient. The tints of close double stars are 

 seldom so plainly seen with low powers as with higher ones, 

 which give a wider separation to the discs. 



Not unfrequently an observer finds considerable trouble in 

 satisfying himself as to the tint of a star. In some cases this 

 probably happens from the want of a standard of white light 

 in the field. Could this be constantly introduced by the 

 method of reflection employed by Chacornac to estimate com- 

 parative brightnesses (seo last number of Intellectual Ob- 

 server, p. 385), it would be a great assistance, and well worth 

 the consideration of an observer who made this subject his 

 special study. In other instances the eye seems puzzled and 

 the tints fluctuate. I have repeatedly remarked this in the 

 smaller components of certain double stars — aPiscium being 

 a remarkable instance — which arc described as blue by Smyth, 

 but appeared to me sometimes of that colour, sometimes tawny, 

 in the course of a single observation. That this is not alto- 



