In 1880 he invented the photophone, which transmitted speech via light waves
He told the 
reporter that the photophone is the greatest invention [I have] ever made, ... of 
Science in August 1880, Bell gave credit to the first demonstration of speech... [DOC] [TLE] Going Wireless In 1880 | Invention and Technology [PAR] In a press release Lucent said that Bell had considered the photophone his 
greatest ...