In an interesting bit of optical sleight-of-hand, researchers at The University of California, Davis have developed inexpensive ways to transform iPhone cameras into reasonably high quality microscopes and spectrometers. 

The modifications involve affixing relatively cheap add-ons to the camera's lens. To make the camera into a microscope they attach a so-called "ball lens", using a kind of rubber band. For the spectrometer, they attach a simple plastic tube covered with perforated black electrical tape.  

The researchers envision the techniques being used to help cheaply diagnose medical conditions in the field, especially in locations with few or inadequate medical facilities.