82 VISION. 
the head of a dragon-fly you will see that even to 
the naked eye they exhibit a grated appearance, such 
as might be presented by a very fine sieve ; and un- 
der a magnifier they show a surface like that of a 
honey comb, consisting of numbers of hexagonal 
compartments. Every compartment is the end of a 
tube, containing a transparent substance like glass, 
its walls lined with black, and its deep extremity 
occupied with a sensory structure continuous with 
the nerve of sight. The effect of this arrangement 
is obvious ; only one ray of light can pass down each 
tube ; all lateral rays being absorbed by the black 
walls. The sensitive structure at the bottom of each 
tube is thus exposed to only one spot in the land- 
scape, the spot directly opposite it; and there is 
painted in the bottom of the eye a miniature repeti- 
tion of the landscape which might be compared with 
a pattern in Berlin wool,each tube containing a single 
stitch. Here then you have undoubtedly an instance 
of true vision—a number of separate spots of light 
appreciated at the same moment, and every spot re- 
ferred, not to its position in the eye, but to the direc- 
tion from which the ray of light has come. This 
sort of eye is an optical instrument which requires no 
focusing. There are just as many points in the 
Jandscape exhibited to the animal as there are tubes 
in the eyes, and if the landscape is distant those 
