What are Slides?
Slides are a transparent media that when light is shown through it, and via some level of magnification, displays the image on a surface for viewing.
History of Slides
Slides in one form or another date all the way back to the 17th century, in what was called a lantern slide (also known as a magic lantern). Lantern slides were a transparent image on glass, and was introduced in 1849. Lantern slides were large compared to today's slides, measuring 3 1/2 x 4", and were fragile since they were on glass.
Today, most people are familiar with 35mm slides, which were invented in 1935.
Common causes of damage
Slides are also vulnerable to heat and humidity, and over time can become brittle and crack, damaging the image.