From one or other of these tables every observable minimum can be obtained. Thus, suppose the observer wants to look for a minimum during the last fortnight in August, 1868. The first table gives him no information, the latter gives him a minimum at 8h. 19m. P.M. on August 30; hence of course there is a minimum at 11h. 31m. P.M. on August 27; and there are no other conveniently observable minima during the fortnight in question. The cause of the remarkable variation in this star's brilliancy has been assigned by some astronomers to the presence of an opaque secondary, which transits Algol at regular intervals; others have adopted the view that Algol is a luminous secondary, revolving around an opaque primary. Of these views the former seems the most natural and satisfactory. It points to a secondary whose mass bears a far greater proportion to that of the primary, than the mass even of Jupiter bears to the sun; the shortness of the period is also remarkable. It may be noticed that observation points to a gradual diminution in the period of Algol's variation, and the diminution seems to be proceeding more and more rapidly. Hence (assuming the existence of a dark secondary) we must suppose that either it travels in a resisting medium which is gradually destroying its motion, or that there are other dependent orbs whose attractions affect the period of this secondary. In the latter case the decrease in the period will attain a limit and be followed by an increase. However, interesting as the subject may be, it is a digression from telescopic work, to which we now return. Within the confines of the second map in Plate 4 is seen the fine star [gamma] Andromedae. At the hour of our observations it lies high up towards E.S.E. It is seen as a double star with very moderate telescopic power, the distance between the components being upwards of 10"; their magnitudes 3 and 5-1/2, their colours orange and green. Perhaps there is no more interesting double visible with low powers. The smaller star is again double in first-class telescopes, the components being yellow and blue according to some observers, but according to others, both green. Below [gamma] Andromedae lie the stars [beta] and [gamma] Triangulorum, [gamma] a fine naked-eye triple (the companions being [delta] and [eta] Triangulorum), a fine object with a very low power. To the right is [alpha] Triangulorum, certainly less brilliant than [beta]. Below [alpha] are the three stars [alpha], [beta], and [gamma] Arietis, the first an unequal and difficult double, the companion being purple, and only just visible (under favourable circumstances) with a good 3-inch telescope; the last an easy double, interesting as being the first ever discovered (by Hook, in 1664), the colours of components white and grey.