Page:Jardine Naturalist's library Entomology.djvu/193

Rh a cornea; beneath this there is a layer of opaque matter, varying in colour, but most commonly black, deep violet, red, or green, which produces the brilliant spots and bands on the eyes of Tabanidæ and other flies; lying below this a dark-coloured varnish, which may be considered as a choroid: numerous air vessels are supplied to the last mentioned part, and there is a space beneath for receiving some of the ramifications of the optic nerve. According to Müller, who has been most successful in explaining the structure of these organs, each individual facet can survey but a small space of the entire field of vision, so that each contributes to the perception of all the objects within the field; but each separate one does not at the same time see all such objects, whence the insect must receive as many forms of objects in its eye, as there are individual facets to the eye. This consequence of a common and yet subsidiary vision of these facets, springs partly from the immobility of the eyes, and partly from the circumstance that only those rays of light which fall in a right line upon a facet of the eye, which itself forms the segment of a circle, can reach the optic nerve of this facet, whereas all others are withheld by the pigment which partly separates the individual glass lenses from each other, and partly surrounds the margin of the chrystalline lens, beneath the cornea. From this it follows, that the nearer the object is the more obliquely do all, but the perpendicular rays of light, fall upon the facet; and, therefore, contribute so much the less to the production of the image; the object consequently is most