If you have vision problems that prevent you from recognizing contents displayed on your computer screen, you can simply use Mousewheel Zoom, which helps users to magnify their computer screen so that they make their font or computer icons and buttons bigger under Gnome Shell.
Once installed, you will be able to zoom in/out your computer screen using your ALT key + mouse wheel (aka scroll wheel). This tool runs only on Gnome Shell and not for Unity. To install it under Ubuntu 12.04/11.10, open the terminal and run the following commands:
sudo add-apt-repository ppa:tobias-quinn/gsmz
sudo apt-get update
sudo apt-get install gnome-shell-mousewheel-zoom
Log out the current session and log back in to enable it. Now use the ALT + Mouse Wheel to magnify your screen. Credits for this tool go here.