Adjustable focus glasses

Adjustable focus glasses are eyeglasses whose lens powers can be adjusted for focal distance and prescription. These glasses are viable options for certain people who need bifocals or trifocals. They can also be donated to people in the developing world where eye doctors and opticians are hard to come by and afford. There's two methods for achieving adjustable lenses. 
One design uses a crystal liquid in a flexible layer. The wearer then uses a dial to control the amount of liquid in the lenses, which in turn makes the lenses more convex, concave or flat.
The other design has double layers of lenses that slide over one another.