EYES AND SIGHT, 
103 
pigment. No optical image is perceived, as each 
facet only gives the image of the object just in front, 
and each cone only receives light from a very small 
field of vision. The picture perceived by the insect 
was in fact a mosaic, and just as we see a pattern in 
mosaic composed of numerous inlaid pieces, so the 
image of an external object is supposed to be made 
up of the separate impressions caused by the rays of 
light proceeding from the illuminated points of the 
object seen, the number of points corresponding with 
the number of facets, the concurrence and combina- 
tion of these separate impressions forming a picture, 
as it were, by the mind s eye. This theory was at- 
tacked by Gottsche (50), who pointed out that each 
separate cornea of a compound eye gives a separate 
and distinct image. Leeuwenhoek (91) had observed 
this, and said: — 
‘ When I removed the tunica cornea a little from the 
focus of the microscope, and placed a lighted candle at a 
short distance, so that the light of it must pass through 
the tunica cornea, I then saw through it the flame of the 
candle inverted, and not a single one, but some hundreds 
of flames appeared to me, and these so distinctly (though 
wonderfully minute) that I could discern the motion of 
trembling in each of them.’ 
This experiment is easily tried, and is described 
in most books on the microscope. The eye of a 
dragon-fly is best for this purpose, if everything but 
the cornea be removed. 
It is impossible for us to enter into the whole con- 
troversy> but we would here point out that when the 
