SIGHT 43 
of its home, and the eyes are under the shadow of the large 
conical shell. Under such circumstances complex visual organs 
are unnecessary. 
Picrure-Eves.—The development of refracting structures in 
direction-eyes has led to the possibility of further specialization in 
vision, and has resulted in what we may 
call Picture-Eyes, capable of giving more 
or less definite information about the form 
and colour of external objects. Two kinds 
of these may be distinguished, ze. Com- _ 
pound Eyes and Camera Eyes. Bee (apie edifice) nace 
Compound Eyes are characteristic of a ircca’ Theancsna ave seen iront, 
larged. The antennz are seen in front, 
great many Arthropods, such as Lobsters 2rd tce spall simple eyes neat 
ases, but the most conspicuous struc- 
and Crabs, where they are placed at the _ tuesare theenormous compound eyes, 
with their minute hexagonal facets. 
end of stalks, and Insects, where they are 
in the form of two large projections on the head (fig. 1056). 
Examination with a lens shows that such an eye is covered by 
a transparent patch of the hard covering of the body, which is 
divided into a multitude of minute square or polygonal areas, 
commonly known as facets. These may be exceedingly numerous, 
as will be seen from the following calculations made by Leeuwen- 
hoek more than a century ago:— 
house-fly, 4000; gadfly, 7000; goat- 
Sess 
moth, 11,000; death’s- head moth, lll 
12,000; swallow-tail butterfly, 17,000; Aig: a rn 
dragon-fly, 20,000; a small beetle SR Va ae 
(Mordella), 25,000. It was origi- ) y a te 
nally believed that these elaborate és 7 
structures were aggregates of simple __ Fis. 10s7—Diagram of a Compound Eye, in 
7 5 section, enlarged, to illustrate theory of ‘mosaic 
eyes, acting independently; and they vision”. Numerous radiating visual pyramids 
“ d ” are indicated, each consisting of external refract- 
were therefore called compoun ing structures (*., 7.) and internal groups of retinal 
‘ : cells (7z.). On the right side part of the pigment 
eyes, a rather misleading term. Sec- of several pyramids is inserted. The course of 
1 light-rays from an external object is indicated for 
tions through such eyes (fig. 1057)  Hzhttey: foe a ee eve 
have demonstrated that each facet 
‘5 the outer end or base of a very slender visual pyramid 
(ommatidium), the external part of which consists of various 
refracting structures, while internally is a group of sensitive 
visual cells connected with nerve- fibres. Adjacent pyramids 
are optically separated from one another by means of pigment. 
Comparison of various compound eyes shows that there are 
