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 avai

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.