Gary Lockwood, a big part of playing the character was being able to wear the silver contact lenses. Once it was discovered that he had trouble with them (Sally Kellerman didn't) it's kind of strange they didn't recast the role.
With the lenses in, he had to tilt his head back to see at all, it made him look like someone with bifocals.
According to These Are the Voyages
(p. 89),this often-repeated story isn't true. It IS true that the lenses hurt Lockwood, but it isn't true that he had to tilt his head back to see though them.
Lockwood is quoted in the book as saying that he tilted his head back because he thought that gave him an appropriately arrogant air, since he was supposed to be playing a would-be god. The head-tilt was an intentional acting decision, not a side effect of the lenses.