212 
Dr. Hosack's Observations on Vision . 
to the too great action of the recti muscles, especially as it is 
of a firm consistence, and the eye rests immediately upon it ? 
Admitting then that this is the operation of the external 
muscles when in a state of contraction, it is also- to be ob- 
serve J, we have the same power of relaxing them, in propor- 
tion to the greater distance of the object, until we arrive at 
the utmost extent of indolent vision. 
But, as a further testimony of what has been advanced, I had 
recourse to the following experiment, which will shew that 
the eye is easily compressible, and that the effects produced 
correspond with the principles I have endeavoured to illustrate. 
With the common speculum oculi I made a very moderate 
degree of pressure upon my eye, while directing my attention 
to an object at the distance of about twenty yards ; I saw it 
distinctly, as also the different intermediate objects ; but en- 
deavouring to look beyond it, every thing appeared confused. 
I then increased the pressure considerably, in consequence of 
which I was enabled to see objects distinctly at a much nearer 
than the natural focal distance ; for example, I held before, 
my eye, at the distance of about two inches, a printed book ^ 
in the natural state of the eye I could neither distinguish the. 
lines nor letters ; but upon making pressure with the specu- 
lum I was enabled to distinguish both lines and letters of the 
book with ease. 
Such then I conceive to be the action and effects of the ex- 
ternal muscles, and which I apprehend will also apply m ex- 
plaining many other phenomena of vision ; some of tnose it 
will not be improper at present briefly to notice. 
First, may not the action of those muscles have more or 
less effect in producing the changes of vision which take place 
