>> before the projector’s heat began to melt the frame.

> What?

The projector uses a hot lightbulb to display the frame in the film onto a wall or screen. The film sits just in front of this bulb, and of course the heat can risk damaging or melting the plastic film if the film is not quickly accelerated past the bulb (as is the case for watching a video - a sequence of frames).

Keeping one frame visible on a wall is to measure the visible size of the fireball, but doing so means one frame stays in front of the bulb long enough to risk it melting.