The Toolglasses cover whatever you are working on, but they don't get in the way because they're transparent. To move the Toolglass around, the PARC guys and gals propose a secondary positioning and input device, such as a trackball, for your non-dominant hand. This sounds like it will work out well, allowing rapid, yet somewhat coarse positioning of the individual lenses. The lenses would be big enough that positioning them in somewhat the right location would be adequate, and the mouse could move in for the fine-grained work within the lens. The Magic Lens user interface is impressive.
Lenses can offer localized views of onscreen objects with surrounding context.
A magnification lens can be moved around on screen, and you can easily see what is around the magnified portion, without needing a whole new window with another small context view. Besides making tools that are easier to use, the Magic Lens project is intended to reduce the time it takes to do operations. This is a noble goal, no matter how you slice it. In many applications, work is done in localized areas, and often the user switches between a small subset of tools. For example, in a paint program, you might choose a tool to pick up a color from the canvas, then zoom into a region and paint a few carefully placed pixels in that color.