337 
THE MODE OF ACTION OF FACETTED EYES. 
By PROFESSOR SIGMUND EXNER * 
I T is generally known that the organ of vision in the animal 
kingdom has been morphologically developed in two direc- 
tions : in one direction to the eye of the vertebrate, in the other 
to the facetted eye. Quite recently, Grenadier’ s anatomical 
investigations have led to the discovery of the common starting- 
point of these two developmental forms, and to the tracing from 
it of the foundation of this sensory organ. *f* That the optical part 
of the eye of a vertebrate throws a picture of the objects in the 
outer world upon the retina, has long been known, and it may 
be demonstrated in the eye of any albino rabbit ; that this pic- 
ture has a physiological significance may be affirmed with cer- 
tainty, for those rays, and only those rays, which produce a 
point of the retinal picture are those which lead to the percep- 
tion of the corresponding points of the object. The picture is 
reversed. 
Affairs are more complicated in the case of the facetted eye. 
Johannes MiillerJ was the first to assert that this eye also has a 
retinal picture, and that it is erect. The mode in which it is 
produced may be explained as follows : — If we imagine the in- 
sect eye deprived of the exquisite transparent structures, the 
cornea and crystalline cone, it may be conceived as a segment of 
a rather thick, hollow sphere, consisting of a pigmented mass, 
pierced by numerous close- set perforations running towards the 
centre of the sphere. If we further imagine the inner surface 
of the spherical segment coated with a membrane, it is clear 
that a picture of external objects must be produced upon the 
latter. Thus, as through the above perforations, only those rays 
can reach the membrane which run in, or nearly in, the axis of 
* Biologisches Centralblatt, Juhrg. I. p. 272. 
t Untersuchungen iiber das Sehorgan der Arthropoden, Gottingen, 1879. 
j Zur vergleichenden Physiolugie des G esichtssinns , Leipzig, 1826. 
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