Great refracting telescope, presented by the emperor Alexander I.
It is a colourless, highly refracting liquid, boiling at 78°; it fumes on exposure to moist air.
In it are, moreover, enclosed unicellular glands pouring their highly refracting contents, of a more or less rod-like shape, directly to the exterior.
The more highly organized species have often very numerous eyes (Amphiporus, Drepanophorus), which are provided with a spherical refracting anterior portion, with a cellular " vitreous body," with a layer of delicate radially arranged rods, with an outer sheath of dark pigment, and with a separate nerve-twig each, springing from a common or double pair of branches which leave the brain as n.
An admissible error of phase of 4X will correspond to an error of IX in a reflecting and 2X in a (glass) refracting surface, the incidence in both cases being perpendicular.