As it would appear, he wasn't invisible after all.
Al looked up from his subject of contemplation to see Andromeda Nott, the new girl at the laboratories. She wasn't around long and she joined them shortly after Caractacus Burke did.
"They're both acceptable," he agreed, glancing up at her and giving her a friendly nod. "I'm glad I'm not the only one to ask for time off in order to enjoy desserts here while the rest are left out to explode the laboratories without our help."