When examining algæ under the microscope, we notice the spaces which separate the streaks ornamenting the silicious covering of these various organisms, and it is explained that they are formed by hexagons visible only when we examine the object with a powerful microscope. “For a long time,” says M. Nachet, “I occupied myself with the examination of the hexagonal appearance of the points constituting the streaks. Why should these hexagons show themselves, and how could they be other than the visible base of small pyramids piled very closely one on the other; and if this were the case, why were not the points of the little pyramids visible? Or, was the structure before me analogous to that of the eyes of insects? Then the carapace would be but a surface of perforated polygonal openings. This latter hypothesis was attractive enough, and would have explained many things; but some careful observations with very powerful object-glasses, quite free from blemishes, had shown me that these hexagons had round points, contrary to the descriptions of micrographs. These observations, corroborated by the micrographic photographs of Lackerbauer, the much-regretted designer, and by Colonels Woodward and Washington, left not the slightest doubt that it was necessary to discover why the eye persistently saw hexagons where there were circles. To elucidate this point, it was necessary to find some means of reproducing artificially what nature had accomplished with so much precision on the surfaces of algæ. After many fruitless attempts, I decided on making a trial of a stereotype plate covered with dots arranged in quincunxes, very close together” (figs. 132 and 133). “The result was more successful than I had hoped; the effect produced is exactly that of the arrangement of the so-called hexagons of the most beautiful of the algæ, the _Pleurosigma angulata_. If these stereotypes are examined with one eye only, we shall be immediately convinced that we have to do with hexagonal polygons.” It is useless to give any long exposition of a figure so clearly explanatory; it is simply an effect of the contrast and opposition of the black and white in the sensation of the retina. This effect is particularly striking with fig. 134, a negative photograph heliographically engraved according to fig. 133. In this the white points seem to destroy the black spaces, and to approach each other tangentially, and the irradiation is so intense that the white circles appear much larger than the black of fig. 133, although of the same diameter. There are in these facts many points which may interest not only students of micrography, but also artists. As to the algæ, the origin of this investigation, it remains to be discovered if these circles which cover their silicious carapace are the projection of small hemispheres, or the section of openings made in the thick covering. Certain experiments, however, seem to prove that