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."