What had to be invented before we could discover Saturn and Neptune?

The telescope should be invented because it look into a space a lot closer than our eyes could.

The discovery of Uranus, Neptune, most asteroids, the moons of

any planet other than Earth, Pluto, and all the other dwarf planets,

required the invention and use of telescopes. Those objects are all

too dim to be seen with a bare naked human eye.

Saturn is bright and plainly visible with your eyes, if you know when

and where to look, and what you're looking at.