The Irlen Institute have provided Flash videos of sample distortions of text to illustrate the issues that may arise with visual stress as have TechDis with Sim-dis where they show how a yellow coloured overlay may be helpful.
“Visual stress (sometimes called ‘Meares-Irlen Syndrome‘ or ‘Scotopic Sensitivity Syndrome‘) is the experience of unpleasant visual symptoms when reading, especially for prolonged periods. Symptoms include illusions of shape, movement and colour in the text, distortions of the print, loss of print clarity, and general visual irritation. Visual stress can also cause sore eyes, headaches, frequent loss of place when reading, and impaired comprehension.” (Lucid Research Limited)
More information on the subject has been written by Professor Arnold Wilkins. Using Coloured overlays, glasses and changing the appearance of webpages and desktop colours on a computer screen can all help.
The W3C Web Accessibility Initiative (WAI) provides a collection of resources on how people with disabilities use the web.