Answer:
It is pronounced as "karl zice," rhymes with "dice," or "rice."
Carl Zeiss was of German ethnicity. In Germany, "ei" is always pronounced as a long "I."
"ie" is pronounced as a long "e," as in "keep."
Besides which, "z" in German sounds "ts", therefore making Zeiss sound as "Tsice". Which along with "Kar-el" for Carl, gives you "Kar-el Tsice".