242 Scientific Proceedings, Royal Dublin Society. 
It is, perhaps, worth observing that an eminently useful 
adjustment which we cannot effect seems to be possible in an 
insect’s eye. In fact, the inner surface of our model might be 
drawn inwards more at one place than another ; and I am disposed 
to think that muscles, acting on the inner surface, are in the insect 
so disposed as to make this possible in its eye. Now, this would 
effect an accommodation to the distances of objects which would 
differ in the different parts of the field of view.’ Moreover, this 
result may be brought about in another way. The lenses and 
the funnels posterior to them vary in size from one part of the 
compound eye of a dragon-fly to another, being largest in the 
position which I suppose to be about the middle of the eye, and 
gradually dwindling to about half this size near the margin.” An 
equable contraction of the “‘inner surface” of such an eye would 
obviously effect a different accommodation in different parts of the 
field of view. Accordingly, in one or other of these ways, or by a 
combination of them both, the insect may be able to adjust one 
part of its field of view for near objects, and other parts for more 
distant ones, ¢.g. a fly may be able to view distant objects around 
with the utmost distinctness of which its eye is capable, at the 
same time that it is closely scrutinizing the details of a lump of 
sugar and applying its proboscis rapidly to one minute crystal 
after another. As the adjustment which would enable it to do 
this would be of service to the insect, and as the construction of 
its eye admits of it, it seems likely that it is one for which 
provision has been actually made. 
On areview of the whole subject we seem to have a satisfactory 
wave-lengths of the light that has to traverse them. Light would adapt itself to the 
sinuosities of such filaments, like sound in a speaking-tube. 
1Tn the sections of the eyes of dragon-flies which I have examined, the filaments 
from the funnels down to the ‘‘inner surface’’ are enclosed within a sheath of fibres 
and are straight, but immediately after passing through the inner surface they are each 
apparently enclosed within a tube, and grouped in bundles, between which are open 
spaces which may, perhaps, in the living insect have been occupied by muscles. Muscles, 
in this situation, would be competent to effect the optical adjustment spoken of in the 
text. (See fig. 3.) 
2 The increased aperture of the lenses towards the middle of a dragon-fly’s eye, and 
the diminished curvature of the stratum in which they lie, both conduce to make its 
vision more perfect towards the middle of its field of view; and as this lies in the 
direction of the insect’s flight, the arrangement must be of advantage to it in its pur- 
suit of prey. 
