The GLens Applet permits you to view images through "lenses" onto a base image. It it used in Sea and Space to permit you to view the single images in a composite image, and to see a map overlaid on the image.
Glens is based on Mark Handel's CLens. The differences are:
All the Sea and Space multitemporal (colour) images are presented with three "single channel" lenses: Red, Green and Blue. Each of these shows a single date in the composite image. So, for example, in the applet for Exercise 1, you can see the movement of the oil slick between the different dates by finding it in each colour lens.
A white lens may also be available. This shows a map of the area, georeferenced to the image. Sometimes the map only covers part of the area, so you will see plain black where there is no map data.
You can control the lenses using the little tabs at the top left and bottom right with the mouse:
Note that while the lens is moving, you will see the base image through it (and you may have to wait a few seconds after moving a lens for it to redraw). You can use this feature to flick between the images: click on a tab to see the base, then release it to see the lens.
GLens may show some basic geographical information in the browser's status bar:
Note that this only works if the applet has geographical information (so it "knows" where the image is); otherwise it will display position in Pixels instead (pixels, of course, have no real-life meaning).