Holmes used a pair of these while on the Police boat opposite Jacobson's Yard, waiting for the Aurora to leave the yard. Holmes had stationed one of the Baker Street Irregulars at the yard to signal when the Aurora was about to leave. (The Sign of Four)

 

Holmes spotted the black mark from Nitrate of Silver on Watson's right forefinger and deduced from that (and from other clues) that Watson was back in medical practice. (A Scandal in Bohemia)

 

While on the way to the Lyceum Theatre, Holmes has his notebook on his knee and is jotting down figures and memoranda.