264 Mr. Smith's Observations on the 
scales, that part of it being similar to the rest of the sclerotica. 
Tendinous fibres were detected spreading over the scales, 
and terminating at last in forming the four recti itiuscles 
belonging to the eye, so that, upon the contraction of these 
muscles, motion of the scales would be produced. This im- 
bricated appearance of part of the sclerotica, and the detection 
of the tendinous fibres spreading over scales terminating at 
last in the four recti muscles, led me to consider 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 ap- 
plied to other animals. 
It is a fact so well known to persons acquainted with op- 
tics that it is almost unnecessary to mention it, that the rays 
of light, passing through a lens, will be refracted to a point or 
focus beyond the lens, and this focus will be less distant in 
proportion as the lens approaches to a sphere in shape. Now 
this principle is very naturally applied to the explanation of the 
use of this apparatus. These scales lying each partly over the 
next, so as to allow of motion, will, on the contraction of 
the recti muscles inserted into, and covering them, move over 
’each other, and thus the circle of the sclerotica will be dimi- 
nished, and of course the cornea, which is immediately within 
the circle made by these scales, will be pressed forwards, or in 
other words rendered more convex, and thus the focus of the 
eye becomes altered, its axis being elongated. This construc- 
tion, and consequent convexity of the cornea, must render small 
objects near the animal very distinct. 
On these muscles relaxing, the elasticity of the sclerotic 
coat will restore the cornea to its original flatness; it thus 
becomes fitted for viewing objects placed at a greater distance 
