58 THE POPULAR SCIENCE MONTHLY. 



frequently contain nonpareil, or letters only a millimetre (o*^ inch) 

 high. 



Many of the text-books required by the school authorities are bad- 

 ly 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 prefer- 

 ence to those establishments which do not use letters of less than two 

 millimetres {-^^ 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 (^ 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 pre- 

 dominate, 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 {-^-^ inch). 



The thickness of the strokes should also be regarded, for it is ob- 

 vious that the form of the letter is more readily and more clearly im- 

 pressed 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 milli- 

 metre (yj-y of an inch), in thickness should not be admitted into school- 

 books. Ample space should be allowed between the letters. Labou- 

 laye recommended that every two lettei^ 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 occa- 

 sions 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 



