Concept:
Crooke's glass is a special glass used for protection from harmful radiations, especially ultraviolet radiation.
Step 1: Ordinary glass may transmit some harmful radiation.
Step 2: Crooke's glass is made by adding certain oxides that absorb harmful rays.
Step 3: Cerium oxide is used in Crooke's glass because it helps absorb ultraviolet rays.
Step 4: Therefore, Crooke's glass contains:
\[
\boxed{\text{Cerium oxide}}
\]