Let's say you have the mesh of the mag.glass.
Create a new camera in the middle of the glass. You can adjust the
FOV/lens to get the right view.
Link both the Camera and the Target to the Glass.
Create the animation you need - the camera will follow.
Hide the mag.glas object and render a flic with the new camera.
Unhide the mag.glas.
Put the ready flic as texture map into the glass material and use planar
mapping coordinates to map onto the glass.
Render the animation from the desired scene camera.
The texture map on the glas will show a magnified view of the scene behind the glass.
You can also get a Raytracer... :o)