frequently contain nonpareil, or letters only a millimetre (1/25 inch) high.
Many of the text-books required by the school authorities are badly printed. The officers should go through every school-book with a millimetre-rule in their hands, and throw out all in which the letters are less than a millimetre and a half high, and should give the preference to those establishments which do not use letters of less than two millimetres (1/13 inch).
The distance between the lines is an important factor in respect to ease in reading. As is well known, the compositors often insert thin leads between the lines so that the letters which project above the average height and those that fall below the line shall not touch. Every one knows that legibility is improved by contrast; the darker the print and the clearer the paper, so much easier is the reading. When the lines are close together, or the matter is printed "solid," the eyes become tired sooner, because the contrast is lessened. The lines tend to run together, and the effort to separate them strains the eyes. In fine editions the lines are widely separated. I consider a book well leaded in which the interlinear space, measured by the shorter letters, amounts to three millimetres (1/8 inch). The lines will really seem to be closer, for the projections of the longer letters will encroach upon the interlinear space; and cases may occur, when those letters predominate, in which the space may seem to be only one millimetre. The narrowest interval that should be permitted is, in my opinion, two and a half millimetres (1/10 inch).
The thickness of the strokes should also be regarded, for it is obvious that the form of the letter is more readily and more clearly impressed on the retina when the stroke is broad and distinct than when it is fine. Letters having a stroke of less than one fourth of a millimetre (1/100 of an inch), in thickness should not be admitted into school-books. Ample space should be allowed between the letters. Laboulaye recommended that every two letter should be separated by a clear space at least as broad as the distance between the two strokes of the n.
Javal believes that the extension of the lines beyond a certain limit of length contributes to myopy, by forcing the eye to endeavor to adjust itself to the varying distances from the eye of the ends and the middle of the line. This has not been demonstrated, but it is not improbable. Every near-sighted person is aware of the pain it occasions him to read a number of long lines without spectacles. The shorter the lines, the more easily they are read, because the eye does not have to make wide excursions. The most suitable length of lines for school-books appears to be about ninety millimetres, or three and a half inches.
Javal has observed that the rectangular Roman letters are liable to be reduced in apparent size, and have their corners seem rounded by
