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.
Besides these more highly differentiated organs of vision, more primitive eyes are present in others down to simple stellate pigment specks without any refracting apparatus.
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.
Benzene is a colourless, limpid, highly refracting liquid, having a pleasing and characteristic odour.
If p" is the refracting angle of the prism, and n the magnifying power of the eye-piece, then p"ln will be the distance observed.
