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caterpillars, should be regarded as “ lenses of very short focal 
length.” He contends that the optic nerve has never been 
satisfactorily traced into the great rods, that the nerve ends be¬ 
neath the great rods in bodies identical in all essential characters 
with the rods of the vertebrate retina, and further that vision 
would he optically impossible if the nerve ended in the great rods 
themselves. He states that the great rods are lenses which 
magnify and reverse the sub-corneal images, and project them as 
a common image on the retina, an arrangement which he claims to 
have discovered. These views have met with opposition, chiefly as 
it appears to me on the ground that nerve-fibres are believed to 
penetrate the great rods, which would be fatal to the new theory. 
We may, however, look forward with interest to the further 
publications of Lowne’s researches on this subject. The question, 
moreover, arises, whether there is any power of lens-adjustment in 
the compound eyes. An object at a distance of more than about 
half an inch from the cornea requires no accommodation owing to 
the minute size of the lens and its exceedingly short focal length. 
The accommodation for objects still nearer is, according to the 
unpublished statements of Lowne, provided for by a perfectly 
intelligible mechanism. 
The swift and safe flight of many insects shows that they not 
only have their wings under complete control, but that they also 
possess excellent visual power, which, so far as locomotion is 
concerned, is not so much needed by slow fliers. Osten-Sacken^ 
points out a relation between the large compound eyes of some 
of the Diptera, and their habit of poising as if to survey the field 
of vision. Such insects rarely walk, and do not require the pro¬ 
tection of the bristles found on other members of the same group 
of flies which have smaller eyes and less power of flight, and walk 
constantly. 
The Eyes of Yerterrata. 
As already stated, the eyes of the Yertebrata are constructed upon 
the same general principles as the simple eyes of insects. The 
vertebrate eye is, however, surrounded by much harder tissue. 
The lens is elastic and is in most cases capable of being adjusted 
to near and distant objects by means of a muscle which surrounds 
its edge. In front of the lens is the iris, which is involuntarily 
dilated or contracted according to the degree of light and the 
distance of the object which is looked at. The iris is only found 
in eyes of Yertebrata. The mode of distribution of the optic nerve 
* 4 Transactions of Entomological Society,’ 1884, p. 497. 
