398 COMPOUND AND FACETTED. 
to produce distinct vision, through the medium 
of a number of minnte facets, or lenses, placed 
at the extremity of an equal number of conical 
tubes, or microscopes ; these amount sometimes, 
as in the Butterfly, to the number of 35,000 
facets in the two eyes, and in the Dragon-fly 
to 14,000. 
It appears that in eyes constructed on this 
principle, the image will be more distinct in 
proportion as the cones in a given portion of the 
eye are more numerous and long ; that, as com- 
pound eyes see only those objects which present 
themselves in the axes of the individual cones, 
the limit of their field of vision is greater or 
smaller as the exterior of the eye is more or less 
hemispherical. 
If we examine the eyes of Trilobites with a 
view to their principles of construction, we find 
both in their form, and in the disposition of the 
facets, obvious examples of optical adaptation. 
In the Asaphus caudatus (see PL 45, Figs. 9 
and 10.), each eye contains at least 400 nearly 
spherical lenses fixed in separate compartments 
on the surface of the cornea.^ The form of the 
* As the Crystalline lens in the eyes of Fishes is spherical, and 
those in the Eye of Trilobites are nearly so, there seems to be in 
this form an adaptation to the medium of Water, which would 
lead us to expect to find a similar form of lens in the compound 
Eyes of all marine Crustacea, and probably a different form in 
the compound Eyes of Insects that live in Air. 
