Page:Sir William Herschel, his life and works (1881).djvu/152

130 affect his results in the way which was striving to avoid.

No research of was more laborious than the elaborate classification of the stars according to their comparative brightness, which he executed during the years 1796 to 1799. It was directly in the line of his main work—to find out the construction of the heavens.

His first paper had been upon the variable star Mira Ceti. Here was a sun, shining by its native brightness, which waxed and waned like the moon itself. This star is periodic. It is for a long period invisible to the unassisted eye. Then it can just be seen, and increases in brightness for a little over a month, and attains a maximum brilliancy. From this it decreases for nearly three months, and after becoming invisible, remains so for five or six months. Its whole period is about 333 days. Are all other stars constant in brightness?