Dr. H. Dor on Vision in the Arthropoda. 231 
J. Miiller considered that each facet, with its conical body and 
the nervous filament attached to it, forms an apparatus which 
transmits to the optic ganglion only the impression of that lu- 
minous ray emanating from an external body which penetrates 
it in the direction of its axis, the oblique rays being absorbed 
by the pigment. The impressions transmitted to the optical 
centre by all the filaments will form a common and continuous 
image. The extent of the field of vision will depend on the more 
or less hemispherical form of the eye; and the distinctness of the 
image produced, upon the length of the cones, the nearness of 
the object, and the number of the facets. The curvature of the 
surface of the cornea, which enabled Leuwenhoeck to obtain 
reversed images with the eyes of insects, is only sufficient, ac- 
cording to Miller, to concentrate the rays in each cone towards 
the point of insertion of the nervous fibre. 
As early as 1759, Porterfield had stated that the compound 
_eyes of insects consist of as many simple eyes as they present 
facets ; that these animals see with their innumerable eyes in the 
same way as Man with two and Spiders with eight, and that the 
number of these organs was destined to replace the movements 
of the eye in Vertebrata. In 1843, Brants adduced the fact, 
already demonstrated by Leuwenhoeck, of the presence of a re- 
versed image corresponding to each facet, but invented a theory 
of interlacement of the nervous fibres in order to bring this fact 
into accordance with the general theory of Miiller. 
Of the authors who endeavoured to combat this theory, one of 
the first was Gottsche, who, in 1852, proved that when the ery- 
stalline body and the cornea are placed together in the micro- 
scope, an image is obtained at the conical extremity of this body. 
His conclusion is that the compound eyes are organs of a nature 
sui generis, and not to be compared with the visual organs of the 
members of other classes of animals. 
In 1855, Leydig put forward a theory which has taken the 
place of that of Miiller. He assumes, with Gottsche, that the 
_crystalline body, its envelope, and the soft layer described by 
' Will between the crystalline and the cornea, are not organs 
corresponding with and attached to the nervous fibres, but ¢hat 
they are the anterior extremities of these fibres themselves, of which 
the form alone is altered, there being no difference between them 
and other parts of the nervous system, either chemically or optically. 
In this view the optic ganglion becomes the analogue of the 
_retina in the higher animals, the crystalline bodies being analo- 
gous to the bacillar coat. Leydig compares the facetted eye of 
an Arthropod to the eye of a vertebrate animal, as follows :— 
“The cornea with its posterior convexity corresponds to the cornea 
and erystalline of the Vertebrata ; the crystalline body (with the 
so-called vitreous body) and the nervous fibre which is attached 
