A further fact which quickly becomes apparent to the naked eye is that the stars are not all of the same colour. Sirius, for example, is of a brilliant white, with a steely glitter; Betelgeux, comparatively near to it in the sky, is of a beautiful topaz tint, perhaps on the whole the most exquisite single star in the sky, so far as regards colour; Aldebaran is orange-yellow, while Vega is white with a bluish cast, as is also Rigel. These diversities become much more apparent when the telescope is employed. At the same time the observer may be warned against expecting too much in the way of colour, for, as a matter of fact, the colours of the stars, while perfectly manifest, are yet of great delicacy, and it is difficult to describe them in ordinary terms without some suspicion of exaggeration. Stars of a reddish tone, which ranges from the merest shade of orange-yellow up to a fairly deep orange, are not uncommon; several first-magnitude stars, as already noted, have distinct orange tones. For anything approaching to real blues and greens, we must go to the smaller stars, and the finest examples of blue or green stars are found in the smaller members of some of the double systems. Thus in the case of the double Beta Cygni (Albireo), one of the most beautiful and easy telescopic objects in the northern sky, the larger star is orange-yellow, and the smaller blue; in that of Gamma Andromedæ the larger is yellow, and the smaller bluish-green; while Gamma Leonis has a large yellow star, and a small greenish-yellow one in connection. The student who desires to pursue the subject of star colours should possess himself of the catalogue published in the Memoirs of the British Astronomical Association, which gives the colours of the lucid stars determined from the mean of a very large number of observations made by different observers. In this connection it may be noticed that there is some suspicion that the colours of certain stars have changed within historic times, or at least that they have not the same colour now which they are said to have had in former days. The evidence is not in any instance strong enough to warrant the assertion that actual change has taken place; but it is perfectly natural to suppose that it does, and indeed must gradually progress. As the stars are intensely hot bodies, there must have been periods when their heat was gradually rising to its maximum, and there must be periods when they will gradually cool off to extinction, and these stages must be represented by changes in the colour of the particular star in question. In all probability, then, the colour of a star gives some indication of the stage to which it has advanced in its life-history; and as a matter of fact, this proves to be so, the colour of a star being found to be generally a fair indication of what its constitution, as revealed by the spectroscope, will be.