With your flat, dry negs out of the sleeves and with emulsion side facing up, place something black under the film, touching the shiny side. If you look at the film from a certain angle the film will look like a very flat positive.
This is similar to how daguerreotypes look positive.
It works particularly well with X-ray film while washing in black trays. I was quite impressed the first time I accidentally observed this. It's how tintypes work.
Yep, that's how tintypes and daguerreotypes work. I first noticed it more than 40 years ago. Works better if the negatives are a little on the thin side.