12 
INSECTS. 
concluded from the appearance of a lateral seetion 
of the cornea, in which the convexities of the 
sides of each lens have appeared similar*. The 
celebrated Swammerdam is of opinion that vision 
is not performed by each lens as a separate eye, 
as in quadrupeds, &c. but that it is excited, as he 
expresses himself, by the mere appulse of the rays 
of light on the lenses, which thus convey the idea 
of the object. He also observes, that under every 
lens is a hexagonal cone of fibres, the base of the 
cone corresponding to the size of each lens. Dr. 
Hook maintains that each lens is convex on the 
external, and concave on the internal side. 
Besides the eyes just described, there are on 
the heads of many Insects three small, smooth, 
lucid globules resembling so many separate eyes, 
placed on the top of the head, between or above 
the lateral ones. These parts Linnaeus distin- 
guishes by the title of Siemmata. Their real na- 
ture is not yet very clearly understood. 
The existence of the Brain in insects is denied 
by Linnaeus, but by this he can only be supposed 
to mean that it does not bear much resemblance 
to that of the larger animals. 
* Some insist that they are so many magnifying Menisci of 
unequal spheres, the exterior or convex part of each being a 
portion of a smaller sphere tliaii the concave or interior part. The 
accurate Roesel represents the supposed convexity of both sides, 
but candidly confesses that he cannot absolutely determine the 
point. The late eminent optician Mr. Benjamin Martin, in an 
ingenious treatise on the nature of vision in Insects, insists 
on their being double convex lenses, and b^s represented the 
proportional convexity in the eye of the Dragon-Fly. 
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