Researchers call the jerky movement saccadic jumps and the pauses that follow fixations . The information is gleaned during the moment of fixation. The jumps traverse the groups of letters and recognize them as the words they are. In other words, we don't read letter by letter, in rows, line by line. Serif Serif fonts assist the eyes movement across a line of text.
Serifs are horizontal extensions of the vertical ends of the lines in the letters.
This is an undeniable advantage of serifs, although you can achieve the same legibility in pother ways, such as increasing the spacing between lines. Leading The space that exists between lines, known as leading, also help the eyes movement across a line of text as well as helping the eye return to the beginning of the next line. Keeping our eyes on this horizontal track is aided by the horizontal direction created by serifs. When lines become too long it becomes increasingly difficult to follow a line of words from left to right without being distracted by the words in the lines above and below. Worse, the longer the lines are, the greater the difficulty of returning to the left-hand edge of the column and finding the next line to read.