During my student days, I was using Herschel prism for Sun observing (it was made by Zeiss) see some info here:
http://www.cloudynights.com/item.php?item_id=109
Due to it's construction (total reflection of light + polarisation), it was possible to adjust the amount of light (white - there was no filtering) that reaches the eye.
However, today Baader Planetarium foil filter is much more practical, inherently safer and way cheaper solution.