438 Colours of Stars. 



even be thought to verge a little toward the opposite, or blue, 

 end of the spectrum. But, whatever probability might attach 

 to Mr. Barker's investigation, the subject seems to have been 

 subsequently neglected; at least, I have not noticed any 

 further reference to it till Herschel II. and South published, 

 in 1824, their catalogue of double stars. In this, remarking 

 upon the smaller star of i Cancri, which Herschel I., 1782, 

 Feb. 8, had found of a deep garnet colour ; Dec. 28, bluish ; 

 1785, March 12, blue, and which they had noted as indigo 

 blue, 1822, Feb. 22, they take occasion to inquire, " Are the 

 colours of the stars liable to change, as well as the intensity 

 of their light ? There is no impossibility in this, and the 

 point merits attention." This it has subsequently received, but 

 hardly, as yet, in the degree which it deserves ; the time,- how- 

 ever, is now obviously come when a more general and rigid 

 investigation may and should be attempted. Fortunately for 

 amateurs, the inquiry is perfectly accessible, in the vast ma- 

 jority of instances, with moderate instrumental means, and, for 

 some not very obvious reason, contrast of colour is frequently 

 as perceptible with small as with larger apertures. And it is 

 an inquiry which calls for an extended combination of effort, 

 for it will be found that it is only by an accumulation of in- 

 dependent and concurrent testimony that we can hope to 

 attain to any reliable conclusion. We have already touched 

 upon this subject at the beginning of our list of double stars 

 (Intellectual Observer, No. 2, p. 148) ; but it is desirable to 

 advert to it again a little more in detail. Many adventitious 

 circumstances are unfavourable to the results of any single 

 observer. From an inherent defect in their construction, 

 achromatic object-glasses do not form an image as perfectly 

 free from colour as the derivation of their name implies ; there 

 is always, under high powers, a narrow fringe of tinted light 

 surrounding every bright object in focus ; and as this tint had 

 been originully a constituent part of the light of the star under 

 examination, previous to its decomposition by the imperfect 

 action of the object-glass, the focal image formed by the 

 remainder of the light cannot be precisely of its natural hue, 

 but must be more or less tinged with (ho complementary 

 colour. By " complementary " colour is meant that which 

 makes up the (xmjn&ment of white light after any given tint 

 has been separated from it: — thus, considering with Sir D. 

 Brewster that white is compounded of certain proportions of 

 the three primary colours, red, yellow, and blue, we shall find 

 that red is complementary to a mixture of blue and yellow, 

 forming green \ that yellow is complementary to a combina- 

 tion of red and blue, forming purple or violet; and that blue 

 is complementary to ;m union of red and yellow, that is to say, 



