When you read you have the impression that your eyes are smoothly moving across the page, but that’s not what is happening at all. Your eyes move in quick sharp jumps, with short periods of stillness in between. The quick jumps are called saccades and the fixations are the moments of stillness. Interestingly ,during the saccades you can’t see anything — you are essentially blind. Fortunately these saccades are really fast so you are not blind for long. They are so fast that you don’t even realize you are having them.

https://www.blog.theteamw.com/2009/12/23/100-things-you-should-know-about-people-19-its-a-myth-that-all-capital-letters-are-inherently-harder-to-read/