He developed the first reflecting telescope, which featured a concave mirror, a flat mirrow & an eyepiece
[DOC] [TLE] 400 years of the telescope - The College of Optometrists [PAR] It became entwined with the certainly much earlier concept of the mirror of ... their 
arrival at the focal surface by means of a concave lens acting as an eyepiece, ... 
for a reflecting telescope in 1663, but the first usable instrument was built to a ...