The 315-Year-Old Science Experiment

A dark filter of some kind ? Can be as simple as smoked glass. The ones manufactured for modern photography are called Neutral Density filters, and they're available in a variety of f-stops, but the intensity reduction is insufficient for observing the sun safely. Higher reduction filters are built for astronomy.

I thought the way that sunspot counting works is that you point a magnifying telescope at the sun, and the image projects on the floor. You look at the image on the floor.

He did have a valid point. For the data to be consistent across a 300+ year data set, he had to use the same method as his predecessors.

However, you could use technology to augment the manual count. They could use the same k-factor adjustment they used to compare multiple human counters, and use the same k-factor adjustment when technology changes.