Certain remarkable groups of stars, of a nature similar to clusters, though not containing such faint or densely packed stars as those we have just alluded to, call for a mention in this connection. The best example of such star groups are the Pleiades and the Hyades (see Plate XX., p. 296), Coma Berenices, and Præsepe (or the Beehive), the last-named being in the constellation of Cancer. Stars which alter in their brightness are called _Variable Stars_, or "variables." The first star whose variability attracted attention is that known as Omicron Ceti, namely, the star marked with the Greek letter [o] (Omicron) in the constellation of Cetus, or the Whale, a constellation situated not far from Taurus. This star, the variability of which was discovered by Fabricius in 1596, is also known as Mira, or the "Wonderful," on account of the extraordinary manner in which its light varies from time to time. The star known by the name of Algol,[32] popularly called the "Demon Star"--whose astronomical designation is [b] (Beta) Persei, or the star second in brightness in the constellation of Perseus--was discovered by Goodricke, in the year 1783, to be a variable star. In the following year [b] Lyræ, the star in Lyra next in order of brightness after Vega, was also found by the same observer to be a variable. It may be of interest to the reader to know that Goodricke was deaf and dumb, and that he died in 1786 at the early age of twenty-one years! It was not, however, until the close of the nineteenth century that much attention was paid to variable stars. Now several hundreds of these are known, thanks chiefly to the observations of, amongst others, Professor S.C. Chandler of Boston, U.S.A., Mr. John Ellard Gore of Dublin, and Dr. A.W. Roberts of South Africa. This branch of astronomy has not, indeed, attracted as much popular attention as it deserves, no doubt because the nature of the work required does not call for the glamour of an observatory or a large telescope. The chief discoveries with regard to variable stars have been made by the naked eye, or with a small binocular. The amount of variation is estimated by a comparison with other stars. As in many other branches of astronomy, photography is now employed in this quest with marked success; and lately many variable stars have been found to exist in clusters and nebulæ.