THE MODE OF ACTION OF FACETTED EYES. 
341 
case. Let a very small, bright point move before the eye of a 
Yertebrate ; the latter will observe the movement, for the 
retinal element (I take only one for the sake of simplicity), 
upon which the picture of the bright point first lay, loses its 
excitation, and in place of it the next one is excited, then, 
again, its neighbour, and so on. It is otherwise in the facetted 
eye. The structure of this is such that the light of the bright 
point penetrates into a whole group of facets, although in 
different quantities. The nerve-element of that facet, in the 
axis of which the bright point is situated, is most strongly 
excited ; the neighbouring facets lying in a circle around this 
receive less light : the circle in contact with these externally 
still less, &c. If now the bright point moves, the excitation 
changes in all these facets, increasing in the one half and 
diminishing in the other. Hence such a movement will be a 
more striking phenomenon for the facetted eye than for the 
vertebrate eye. What is shown here in the case of a point 
of course applies to any moving object. 
My notion, therefore is, that in the seeing of movements 
the facetted eye is superior to the eye of the vertebrate animal, 
but inferior to it in distinguishing objects — that is to say, in 
acuteness of vision. 
A further investigation upon this subject is due to Oskar 
Schmidt.* In various Crustacea and insects this observer found 
crystalline cones, which are by no means cones in a geometrical 
sense, but rather bent after the fashion of a horn. This was 
more particularly studied in Phronima. In consequence of this 
observation Schmidt also turned against Gottsche’s conception 
of the eye, and in point of fact how should a picture be pro- 
duced by refraction at the bottom of a bent cone ? 
Schmidt, however, regards his new observations as also 
incompatible with the other theory. He says: ‘Nevertheless 
“mosaic vision 55 is also excluded in the case of Phronima , for 
the pre-supposed conditions for “mosaic vision/ 5 straightness 
of the axis of the refractive body, and absorption of the rays 
which enter laterally, do not occur here. 5 
I believe that 0. Schmidt’s interesting discovery contains a 
confirmation and not a refutation of the theory of ‘ mosaic 
vision, 5 for it is only compatible with this. The straightness of 
the axis of the cone is not in my judgment a necessary con- 
dition of ‘ mosaic vision, 5 and the absorption of laterally pene- 
trating rays is not prevented by the curvature of the cone. 
Imagine a cone with a straight axis, at the apex of which, 
therefore, as above explained, the rays which come approxi- 
mately from the direction a, are brought together. If we now 
* Zeitschr. far wiss. Zool. Ed. xxx. Suppl. 
