EYES OF INSECTS. 207 
Lucy, It is thicker in the middle^ 
than near the edge. 
Mother. Any surface, that gradually 
rises from the edge towards the centre, 
is termed convex : a glass of this form is 
called a convex lens, and it magnifies 
more or less, in proportion to its con- 
vexity. 
Lucy. That is, its greater or less 
tliickness in the middle, is it not ? 
Mother. Precisely. By means of mi- 
croscopes, or lenses, which magnify con- 
siderably, it has been discovered, that 
the eye of a dragon fly, and of most 
other insects, is composed of a num-- 
ber of very small convex lenses, 
which are so combined as to form, in 
appearance, one single projecting hemis*- 
phere. Do you remember the meaning 
of that word ? 
Lucy. Oh yes, I do ; a sphere is a 
globe, and hemisphere is a half globe. 
Mother. Now each of these very 
small lenses separately transmits what- 
ever object is directly opposite to it, and 
