Sclera

The sclera is the opaque (usually white), fibrous, protective layer of the eye containing collagen and elastic fibers. In children, it is thinner and shows some of the underlying pigment, appearing slightly blue. In the old, however, fatty deposits on the sclera can make it appear slightly yellow.

Diseases and disorders
There are some diseases like scleritis that cause partial blindness. These diseases produce a dark vertical line through the eye sight which cause less than 100% visibility.