150 Double Stars. 



beautiful blue tint. Instances of a ruby colour are to be found 

 in the heavens, but only in solitary and not large stars. 



5. Variation of light. — The experience of later times has 

 shown the wide extent of this marvellous phenomenon, and 

 every year is adding to the probability of the worthy old 

 Hanoverian astronomer Schroter's idea, that variable light is 

 present throughout the whole creation. It will be readily seen 

 how favourable an opportunity is offered for its detection in 

 nearly equal pairs, where juxtaposition gives an admirable test 

 of change. But a more surprising and still more unintelligible 

 development of this peculiarity comes out of the Dorpat obser- 

 vations. Struve has discovered what he thinks satisfactory 

 evidence of alternate variation, each component taking prece- 

 dence in turn, in 23 pairs, and has suspected it in 42 more. It 

 may be remarked that it must be difficult to ascertain, in many 

 cases, whether this variation is extended to both individuals, or 

 is confined to one, as the relative effect would be the same upon 

 either supposition; and it remains to be seen whether these 

 alleged changes are equally apparent to other ages ; all of them, 

 at any rate, have not been so, but the subject is too interesting 

 to be passed by lightly, and, as in the case of colours, it is from 

 an average of not only many observations, but many observers, 

 that these very delicate and almost evanescent data can be de- 

 termined. The amateur who is not provided with a telescope 

 equal to those of Struve and Smyth, need not despair, as, from 

 some cause not yet clearly explained, but connected probably 

 with the sensitiveness of the retina, inequality of light is found 

 to be more perceptible when its absolute quantity is diminished 

 in the use of a smaller aperture. 



To this enumeration of the principal points to be noticed, 

 we must add that the observer should record the epoch of his 

 observation. This may prove not merely interesting to himself, 

 but of more general value. Even a negative statement — as of 

 apparent permanency — is of use if followed by positive change. 

 ■ Our readers may not have the means of executing those delicate 

 measurements which decide the periods of revolution, but they 

 may feel much interest in watching, year by year, the diminution 

 or increase of distance, or progressive change of angle, which 

 attests the fact ; and memory, in such cases, is seldom to be 

 fully depended upon. Astronomical dates are frequently given 

 in decimals of a year, rather than in months and days; thus 

 1862-49 signifies June 30 of the present year. This mode of 

 reckoning maybe found in the NauUcal Almanac ; it has the 

 advantage of conciseness, and ranges much better in column. 



It may be well to remind the commencing student that as 

 few, comparatively, of the planes of the orbits of these binary 

 stars lie at right angles to our line of vision, they are for the 



