NATURAL 
Observations on the Structure of 
the Eyes of Birds, by Mr. Pierce 
Smith, Student of Physic ; from the 
Philosophical Transactions. 
\ \ J HILE examining the eyes of 
Birds, I observed in them a 
‘singular structure, which I believe 
has not been hitherto noticed ; and 
though not the object I had in view, 
_in the examination, it will perhaps 
elucidate several remarkable cir- 
cumstances in the natural history of 
these animals, and may ultimately 
be applied to the eyes of other ani- 
mals and add one additional dis- 
covery to those already made on 
this beautifully constructed organ. 
In March, 1792, I observed, 
while dissecting the eyes of birds, 
an irregular appearance of the 
sclerotica, in that part of it which 
immediately surrounds the cornea ; 
and which in them is generally flat. 
On a more minute examination, it 
appeared to be scales lying over 
each other, and which appeared 
capable of motion on each other. 
These appearances I shewed to Dr. 
Fowler, of London, and likewise 
to Mr. Thomson, surgeon, Edin- 
burgh. In June, this paper was 
copied out, at my request, by Mr. 
Irving, who resided jn the same 
house with me. On investigating 
this singular structure, the scales 
were found to be of tony hard- 
: hay : ae #59 A} . oF ‘ i; 3 
HISTORY: 
ness, at least much more so than any 
other part of the sclerotica. On the 
inside of the sclerotic coat of the 
eye there was no appearance of 
these scales, that part of it being 
similar to the rest of the sclerotica. 
Tendinous fibres were detected 
spreading over the scales, and ter- 
minating at last in forming the four 
recti muscles belonging to the eye, 
so that, upon the contraction of 
these muscles, motion of the scales 
would be produced. ‘This imbri« 
cated appearance of part of the 
sclerotica, and the destruction of 
the tendinous fibres spreading over 
scales, terminating at last in the 
four recti muscles, led me to con» 
sider the use of this structure, what 
would be the effect of motion of the 
scales upon the vision of birds, and 
how far this'can be applied to other 
animals. 
It is a fact so well known to per« 
sons acquainted with optics, that it 
is almost unnecessary to mention it, 
that the rays of light, passing through 
alens, will be refracted to a point 
or focus beyond the lens, and this 
focus will be less distant, in propor 
tion as the lens approaches, to a 
sphere in shape.. Now this princi- 
ple is very naturally applied to the 
explanation of the use of this ap 
paratus. These scales lying each 
partly over the next, so as to al-' 
low of mojon, will, on the cons 
traction 
