Context: The Scheinergrade (Sch.) system was devised by the German astronomer Julius Scheiner (1858–1913) in 1894 originally as a method of comparing the speeds of plates used for astronomical photography. Scheiner's system rated the speed of a plate by the least exposure to produce a visible darkening upon development. Speed was expressed in degrees Scheiner, originally ranging from 1° Sch. to 20° Sch., where an increment of 19° Sch. corresponded to a hundredfold increase in sensitivity, which meant that an increment of 3° Sch. came close to a doubling of sensitivity.

Question: Who created the Scheinergrade system? 
Answer: the German astronomer Julius Scheiner

Question: What is the Scheinergrade system used for?
Answer: comparing the speeds of plates used for astronomical photography

Question: What do degrees Scheiner indicate?
Answer: the speed of a plate

Question: What is the range of Scheiner speeds?
Answer: from 1° Sch. to 20° Sch

Question: Approximately how many degrees indicate double the sensitivity? 
Answer: an increment of 3° Sch

Question: What was the second use of the Scheinergrade system?
Answer: Unanswerable

Question: What did an increment of 1 degree Sch. correspond to?
Answer: Unanswerable

Question: What was 20 degrees Sch. indicative of?
Answer: Unanswerable

Question: What measured the visible darkening of a developing photograph?
Answer: Unanswerable

Question: When did the Scheinergrade system become outdated?
Answer: Unanswerable