In olden times we had lots of tricks up our sleeves. We would mount negatives on a viewing screen. This is milk-glass illuminated from behind (X-rays are viewed this way). We then took a picture of the negative. The result was a positive suitable for projection. In truly olden times, these were called “lantern slides” because ancient projectors, before electricity, were illuminated using kerosene or gas lamp.

When copying films or prints, the resulting image was likely too contrasty. This is likely because you always lose some of the tonal range when making a copy. We used low-contrast film to mitigate. We had special “copy film”. For color we had “internegative” film. We even had “direct-positive” materials. These made copy slides and copy prints avoiding the internegative step. Let me add that these specialized films and papers delivered optimum quality results.

In a pinch, we used ordinary film to make copies. Today, you can image prints or slides or negatives via the camera or by scanning and using available software to make positive or negative images. Today it’s laid-back; in olden time it was a coup to make faithful copies or reversals.