Joseph Bertrand

lived from 1822 to 1900

Bertrand was a Paris professor who, in 1845, conjectured:

I've told you once and I'll tell you again
There's always a prime between n and 2n.
 
This was proved by Chebyshev in 1850.

Find out more at:
http://www-history.mcs.st-andrews.ac.uk/Mathematicians/Bertrand.html