LXXXVI
§ 210. Although objects standing at various distances must
produce images appearing at various distances behind the crys
talline lens, in a good eye we always see the image falling upon
the retina; hence the eye must have the capacity to change the
convexity of the crystalline lens, which faculty is called the
power of accommodation. 388.
§ 211. The sensation of light must last for some time, else
it will not be felt; f.e., We cannot see the bullet fired from a
cannon. 387.
§ 212. On the other hand we experience the sensation of
light also some time after the impression has been made; f.e.,
a glowing coal rapidly turned round produces a circle of light,
the wheel of a carriage rapidly turned round, we cannot see
the spokes (the space seems to be filled with a half transparent
matter); looking for some minutes at the sun whilst setting
and then turning away our eyes, we see for some time several discs
of the sun-why several? 387.
§ 213. The sensation of light is felt not only on that part
of the retina, which was struck directly by the light, but also on
the surrounding parts; this is called Irradiation; f.e., the cres
cent seems to belong to a larger circle than the ashen-hued not
illuminated portion of the moon's disc. Bright stars appear
much larger than they really are.
§ 214. That part, in which the optic nerve enters the eye
is unsusceptible to light (punctum coecum).
§ 215. The membraneous cataract is a darkening of the
crystalline lens, amaurosis is a hebetation of the optic nerve; the
first disease may be cured by removing or lowering the lens,
the second is incurable. Although the lens has been removed,
people can see, why? but they are long-sighted, why?
§ 216. As to the mechanism of vision, vision is a sensation
