areolæ

By careful adjustment of the focus of the instrument, with a power proportioned to the size of the areolæ, the granules can be seen in the centre of the hexagons ; care must, however, be taken not to confound an optical effect with the appearance of the granules ; each areola is a minute lens, and so refracts the light as to give a bright or dark dot as the focus is changed, and the granules themselves contribute to this effect.

1865, Charles Stodder, Proceedings of the Boston Society of Natural History, volume IX, page 4