This is a simple hand-held spectroscope designed around standard 2" x 2" interference grating slides. The spectroscope has a slit for inserting the grating slide--if it doesn't fit, just sand the slit. You may also need to sand the main optical slit or clean it up with a razor.

There are two ways of making it. The cheaper way is to use a cardboard tube and just print two end-pieces. I used a paper towel roll. The more expensive way is to print it all in one piece.

To use it, point it in such a way that the light source lights up the slit and the light goes straight down the middle of the tub. Then look through the eye-hole, which is at a 30 degree angle to the light. Do NOT point at the sun.