Where did slide shows come from?
Has to be where "Slide Shows" came from!
Slideshows were in existence as early as the 17th century. They comprises of images which were projected onto walls and were backlit by candles. Nevertheless, it was the Langenheim brothers, who initiated the placing of negatives on glass slides. You can get detailed information at http://www.artinti.com/articles/slideshow_maker/the_history_of_slideshows.