Trying to read an iBook or Kindle eBook at night and curious how to switch to white text on a black background so as not to strain your eyes or keep others awake from the glare? Luckily iPhone, iPod touch, and iPad have this feature built right in. We'll show you how to enable it after the break!

Because "White on Black" is considered a system-wide feature, so all we need to do is head into Settings.

• Open Settings
• Tap General
• Select Accessibility
• Switch "White on Black" to "ON"
• Exit settings, launch iBooks or Kindle, and enjoy!

Bonus tip: If you want to have white on black as a quick option then place it on your triple click home button.

• Settings
• General
• Accessibility
• Triple click home
• White on Black

There you have it! Now can read your books (or web pages, or email, or anything really) as "White on Black" for those lighting situations that require it. If you have an questions, let us know in the comments below.

Tips of the day will range from beginner-level 101 to advanced-level ninjary. If you already know this tip, keep the link handy as a quick way to help a friend. If you have a tip of your own you'd like to suggest, add them to the comments or send them in to news@tipb.com. (If it's especially awesome and previously unknown to us, we'll even give ya a reward...)

Speaking of iPhone 3GS accessibility features, it looks like Apple is improving them in iPhone 3.1 Beta 2 by adding a new "triple click" option to the home button to toggle VoiceOver, Toggle White on Black, and Ask (which then will pop up a menu offering Turn VoiceOver On, Turn Zoom On, Turn White on Black On).

Kudos to Apple for not only doing a great job implementing the various accessibility features in iPhone 3GS, but for continuing to improve them via forthcoming firmware.