That this is not a necessary characteristic of light was discovered by Christian Huygens, who found that, whereas a stream of sunlight in traversing a rhomb of spar in any but one direction always gives rise to two streams of equal brightness, each of these emergent streams is divided by a second rhomb into two portions having a relative intensity dependent upon the position with respect to one another of the principal planes of the faces of entry into the rhombs - the planes through the axes of the crystals perpendicular to the refracting surfaces.
The stereom shows no trace of canals, folds, rhombs or diplopores.
The stereom of the thecal plates may be thrown into folds, but the mesostroma does not so much tend to lie in strands traversing the sutures, nor are pectini-rhombs or porerhombs developed; diplopores are always present in the mesostereom, but often restricted to definite tracts or plates, especially in higher forms. Families: Sphaeronidae; Glyptosphaeridae, e.g.
Four series of "Researches on Heat," in the course of which he described the polarization of heat by tourmaline, by transmission through a bundle of thin mica plates inclined to the transmitted ray, and by reflection from the multiplied surfaces of a pile of mica plates placed at the polarizing angle, and also its circular polarization by two internal reflections in rhombs of rock-salt.
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