Grab a compact disc (scratched ones that won’t play work just fine for this) and center it on the hole mark. As you drill the hole, watch the reflection in the CD—keep the bit in a straight line with its reflection for a perpendicular hole every time.
Cell phone to microscope conversion TUTORIAL. I love the idea that scientists are looking more into the advantage of a cell phone in lab settings. This is an old instructable and I remembered always wanting to do it.