442 
COMPOUND EYES OF ARTICULATA. 
correspond in function with the single eye of the Vertebrate 
animal. For no rays except those which correspond in direc¬ 
tion with the axis of each cone, can reach the fibre of the 
optic nerve at its apex; all others being stopped by the layer 
of black pigment which surrounds it. Hence it is evident 
that each separate eye must have an extremely limited range 
of vision, being adapted to receive but a very small collection 
of rays proceeding from a single point in any object; and as 
these eyes are usually immoveable, animals with but a small 
number of them would be very insufficiently informed of the 
position of external things. But by the vast multiplication in 
the number of the eyes, and the direction of their axes to 
every point in the hemisphere, their defects are compensated; 
a separate eye being provided, as it were, for every point to 
be viewed. And it is quite certain, from observation of the 
movements of Insects, that their vision must be very perfect 
and acute. 1 
575. Although these Compound Eyes exist in all Insects 
and in most Crustaceans, Spiders and Centipedes, they are in 
general not the only organs of vision which these animals 
possess. Most of them are also furnished with several simple 
eyes, analogous in their structure to those of higher animals, 
but less complex and perfect in their organization; these, 
which are for the most part disposed on the back of the head, 
are largest in Spiders. The larvae of some Insects possess the 
simple eyes without the compound; the latter being only 
developed at the time of the last metamorphosis. The simple 
eyes of Insects do not appear to be nearly so efficient as 
instruments of vision, as are their compound ones; for when 
the latter are covered, the animals seem almost as perplexed 
as if they were perfectly blinded. Simple eyes, closely re¬ 
sembling those of Insects in structure, are found in most of 
1 It is commonly believed that each of these compound eyes pro¬ 
duces its own image of the same external object, as do our two eyes; 
but from the description here given of their separate directions when 
united, it is evident that in no two of them can an image of the same 
object be formed at the same time. The membrane formed of all the 
lens-like corneae united together, when separated from the other parts 
of the eye, and flattened-out, has the properties of a multiplying-glass, 
each lens forming a distinct image of the same object; but this is not 
the case when they are arranged in their natural position, because no 
two of them have the same direction. 
