on the Mechanism of the Eye. 81 
and by Mr. Warren*, afterwards in two excellent Memoirs of 
M. Petit on the eye of the turkey and of the owl,-f and lately 
by Mr. Pierce Smith, J and Mr. Home,§ can, on any suppo- 
sition, have but little concern in the accommodation of the eye 
to different distances : they rather seem to be necessary for the 
protection of that organ, large and prominent as it is, and un- 
supported by any strength in the orbit, against the various acci- 
dents to which the mode of life and rapid motion of those ani- 
mals must expose it ; and they are much less liable to fracture 
than an entire bony ring of the same thickness would have been. 
The marsupium nigrum appears to be intended to assist in 
giving strength to the eye, to prevent any change in the 
place of the lens by external force : it is so situated as to inter- 
cept but little light, and that little is principally what would 
have fallen on the insertion of the optic nerve ; and it seems to 
be too firmly tied to the lens, even to admit any considerable 
elongation of the axis of the eye, although it certainly would 
not impede a protrusion of the cornea. 
With respect to the eyes of insects, an observation of Pou- 
part deserves to be repeated here. He remarks, that the eye 
of the libellula is hollow ; that it communicates with an air- 
vessel placed longitudinally in the trunk of the body ; and that 
it is capable of being inflated from this cavity : he supposes that 
the insect is provided with this apparatus, in order for the 
accommodation of its eye to the perception of objects at different 
distances. || I have not yet had an opportunity of examining 
* Phii.Trans. Vol. XXXIV. p. 113. Abr. Vol. VII. p. 437. 
f Mem. del’Acad. 1735. P- l( > 3 ' l 73 ^> P- 166. Ed. Amst. 
t Phil. Trans, for 1795. p. Z63. § Phil. Trans, for 1796. p. 14. 
|j Phil. Trans. Vol. XXII. p. 673. Abr. II. p. 762. 
M 
MDCCCI. 
