Carbon Arc Projectors - This is what we used to do.

He doesn't mention it, but if you watch as he lights the carbon arc he is looking up, not through the glass. the glass is dark, like welding goggles and until the arc lights you can't see through it. What he is watching is the target. The white disc top left of this machine. (Location varied by machine make). The target is actually a piece of white cardboard with a target on it, the light from the arc lamp shows up through and opening in the machine and is seen on the target. The paper target had to be changed as it would receive a scorched image. The target had to be checked from time to time because the rods burned down and had to be manually adjusted. The machines I operated had angled rods, one rod was straight, the other went in at about a 45 degree angle.

Projectionists used to have to be licensed, this was issued by the State Fire Marshals office. Early film was highly flammable so the projectionist had to know what he was doing. The older film was so flammable that the projectionist was expected to be expendable. The Booths were concrete and cinder blocked, the door to the booth was steel, and above the projection windows were steel plates attached to wires or rods with a lead heat joint. The heat from ignited film was explosive and the intense heat would melt the lead quickly, releasing the links which made the heavy shutter doors drop sealing the booth and not allowing the flames to enter the theater.

After Carbon Arc came the Xenon Arc bulb, which lasted a lot longer than the rods, and did not require exhaust pipes on the machines

They did on occasion explode though making a mess.

Film feed was also changed, in the old days movies were shipped on shipping reels and put together (spliced) onto larger reels which went on the machines. Since an entire movie would not fit on one reel, two machines were used, this called for a change over. If you watch an old movie you will notice a white flash in the top right corner occasionally, these are change over marks. When the film got to a certain point on the reel a bell would ring, the projectionist would move next to the second machine and watch out the window, the white flash would appear, on the second flash he would throw the light to the second machine. Since both reels had a little over lap there would be a couple seconds of dual running machines. The projectionist would then remove the bottom reel and the top reel on the first machine and rewind the film. Some movies were long enough to require multiple change overs...Doctor Zhivago comes to mind.

Change overs came to an end with the advent of the film table system, the small shipping reels would be spliced together onto a table, the entire movie would be on one "Platter".  Once loaded the receiving film disc, a guide which the film loaded against would be removed. Through a series of pulleys the film would feed out of the center of the platter up the side of the stand across to the projector, through the machine, down to a pulley, across the room to the stand and onto a second platter. The feed and retrieving pulleys had spring dampers to eliminate shock or film breaks and allow the film to load to the table at a smooth tension.

At least that was how it usually worked, a failure of a tension spring roller or a shift in the outer film stays (small removable L brackets which held the film from sliding), could make the projectionist wish he had that night off.

The third platter allowed for another feature (film) to be ready for threading up and running.
Since the beginning of the film was always loaded and unloaded from the center, rewinding was only done when the movie was broken up onto the small reels for shipping.