<human>:  When did the U-boats start to use the periscope?
<bot>: The periscope was invented in 1849 by a French inventor named Armand Fizeau. The periscope was initially used in naval warfare to improve the accuracy of artillery fire, but later the periscope was used in submarines. The first submarine to use the periscope was the German U-9, which was built in 1884.