Found a box with old family negatives, and found that I could view them by using the Ipod’s camera, and running a camera effects app, in this case, 100 FX. Among the effects is one called “x-ray” which reverses everything, simulating a negative. A lot of camera apps have this feature. The net effect is the picture being viewed is seen as it would be if printed.
This is probably old hat to most, but I thought it was pretty cool 
You can do it with color too, by the way.