These minimal forms as stated by Burgerstein and Netolitzky should be made requirements, except that possibly the distance between letters is not so important as they urge. The minimum of six or seven letters per running centimeter is a convenient approximate gauge which can be quickly applied and is not too stringent.
Griffing and Franz found that legibility increased somewhat, though not greatly, with increase in the distance between the lines, with the leading, as it is called. Cohn thinks it important that there should be a minimum interlignage of 2.5 millimeters, and Sack requires the same. Javal does not find that interlignage increases legibility appreciably, and thinks that the space used for interlignage had far better be given to an increased size of letter without interlignage. The leading is doubtless a mistake when the size of type is below the requirements made above. The size of type should by all means be increased instead, as this is by far the most important of the factors conditioning fatigue. However, a certain amount of leading should be required in school books, at least, but hardly more than Cohn's minimum of 2.5 millimeters.
As to length of lines there is a general consensus in favor of the shorter as against the longer lines, with a tendency to favor 90 millimeters as a maximum, some placing the maximum at 100 millimeters. The latter is doubtless too high. Javal, who has studied the matter very carefully, insists that the maximum should be considerably below even 90 millimeters. He names as one of the principal causes of fatigue in reading, and as a cause tending to produce and aggravate myopia, the considerable amount of asymmetrical accommodation required as the eye moves along a long line, the amount increasing always with the length of the line. Even with the page squarely before the reader, unless he makes constant and fatiguing movements of the head while reading, the reading matter is always farther from one eye than from the other, except at a middle point, and the reader strains to accommodate for both distances, especially for objects held so near as is the page in reading.
Against the long lines is also to be urged the difficulty and distraction incident to finding the place at each turn to the next line, increasing always as the lines are longer. Besides, the longer lines require a greater extent of eye-movement for a given amount of reading. This comes from the fact, verified by various experimenters, that the eye does not traverse the whole line in reading, but begins within the line and usually makes its last pause still farther within, reading the first and last parts of the line in indirect vision. The amount of this indentation tends to be a constant amount irrespective of the line's length, and is consequently a larger proportion of the line's length in the shorter lines. There is thus an important lessening of
