A cataract is a clouding of the eye's natural lens, which lies behind the iris and the pupil. The lens works much like a camera lens, focusing light onto the retina at the back of the eye. The lens also adjusts the eye's focus, letting us see things clearly both up close and far away.  (Reference:  http://www.allaboutvision.com/conditions/cataracts.htm)

I believe canary cataracts are of the nuclear cataract type.

more to come...