O'SHEA — ASPECTS OF MENTAL ECONOMY. 183 



pation of nervous force. Cohn, 1 Sanford, 2 and others have cal- 

 culated the size and shape of letters which can be read with 

 greatest ease. Burnham 3 summarizes the results of investiga- 

 tions upon this subject in an excellent way and his words may 

 be quoted : 



"1. Size of the Letters. It is necessary that type be easily legible at a 

 distance of twenty inches. For this it is necessary, according to Dr. 

 Cohn's estimate, , that the letters be 1.5 mm. high. 'Any type that is 

 smaller than 1.5 mm. is injurious to the eyes.' An angle of five minutes 

 is sufficient for the recognition of a letter; hence, according to the es- 

 timate of Dr. Weber, a healthy eye can read clearly a letter .7 mm. in 

 height without extreme convergence; but such reading is very laborious, 

 Weber tested the time occupied in reading with different types. When 

 the size of the letters was greater than 2 mm., he found that the speed 

 of reading was retarded. He decided for a minimum of 1.5 mm. Eulen- 

 berg and Bach demand still larger type for the lower classes. Many 

 of the school books in common use have much smaller letters than 

 Weber's minimum. In some of the school atlases, letters only .5 mm. 

 in height are used. Many very beautiful maps also have very small 

 type. Especially before pupils have become familiar with the letters, 

 it is necessary to have large type. Javal wishes to have it determined 

 by experiment how large the type should be in the different classes in 

 order that no pupil, however bad the light, need bring his eyes too near 

 the book. 



"2. The Thickness of the Letters. 'No print of which the down stroke 

 is thinner than 0.25 mm. should be tolerated in school books.' In Latin 

 type the corners of the letters should be strengthened in order to make 

 them appear rectangular, otherwise by the irradiation of the white 

 ground they appear rounded off. 



"3. The Shape of the Letters. Javal points out that, if the lower half 

 of a series of words be hidden from view, it still is easy to read them. 

 But when the uppe.- half is covered, it is difficult, often impossible, to do 

 so. In reading, we glance along the line at a little distance above the 

 center of the letters; and the letters g, j, p, q and y are the only ones 

 that come below the line. Thus the upper part of the letters are of 

 special importance. As the result of an extended research, Dr. Sanford 

 found several groups of letters in our alphabet that are relatively poor. 

 Of the small letters c, e, o are conf usable with one another; a, n and u 

 are poor letters; an d s, i, 1, t are liable to cause confusion. Javal, Cat- 



1 Op. cit., Chap. XXIII. 



2 The Relative Legibility of the Small Letters, Am. Jour, of Psychology, Vol. L 

 pp. 402-435. 



8 Op. cit., pp. 49-51. 



