For this article, I have to thank Cathy’s cat and my optometrist.

Yesterday, I had my eyes checked. As is usual for such occasions, I had my eyes dilated, and thus had trouble afterward seeing much of anything, let alone a computer screen. After an hour or so, the blurriness and light-sensitivity had diminished, but not quite enough.

I was able, though, to make a quick temporary change to my computer to make things a) darker and b) still high-contrast so I could read the screen, though still with a bit of a squint.

On the Mac, you can get a photo-negative effect by invoking “white-on-black”. Either open System Preferences ->Universal Access -> Seeing and turn on white-on-black , or use the keyboard shortcut control+option+command+8 to toggle back and forth. It doesn’t just make text come up as white-on-black – it reverses the whole color-scheme, so everything looks exactly like a photographic negative.

On Windows XP, you can get a similar effect by using the Accessibility Options control panel, then selecting the Display Tab, then the High Contrast settings.
Here, you can choose a white-on-black or a black-on-shite high-contrast scheme.
Once you turn on the keyboard shortcut here, you can toggle high-contrast by pressing left-alt + left-shift + prtscrn.

If you’re wondering where the cat comes in, read on.

I received an email from Cathy, one of my clients. It said something along the lines of “when I picked up my cat (off the keyboard), the screen was weird…” I’ve had this call enough times that I was able to guess the symptom, and provide a quick cure, right off the bat.

The moral of the story? Don’t let cats compute.

For further (rather dry) reading, Microsoft has a tutorial that has step-by-step instructions. Apple has a page detailing the accessibility tools available on OS X , and some help pages listing available accessibility keyboard shortcuts and preferences.