The retina is a comparatively thick membrane composed of eight layers of
different kinds of nervous tissue. The essential layer, that of the
“rods and cones,” is the seventh from within outward. Thus a ray of
light on entering the eye must pass through six superficial layers
before it reaches the “rods and cones.”
The “rods and cones” are lying on a layer of colored or pigment cells
whose duty it is to prevent diffusion of light within the eye. The
eyeball, therefore, is to all intents a camera obscura, the iris
representing the shutter, the crystalline lens the camera lens, and the
layer of “rods and cones” the sensitive plate. When a ray of light falls
on the layer of the “rods and cones,” this layer receives a nervous
stimulus which is conveyed by the optic nerve to the brain. _It is these
sensations which the brain translates into what we term sight._
Where the optic nerve enters the back of the eye, there are no “rods and
cones,” hence rays of light falling on this portion of the retina send
no stimulus to the brain; in other words, images falling on the “blind
spot” are not visible.
The “yellow spot” is a small area at the center at the back of the eye
where the retina is very thin, consisting of little more than a single
layer of “cones.” Images which fall upon this region are seen with the
greatest distinctness.