Sentence:  I believe that letters which are less than a millimetre and a half (1/17 inch) high, will finally prove injurious to the eye. How little attention has hitherto been paid to this important subject is exemplified in the fact that even oculistic journals and books frequently contain <WSD>nonpareil</WSD>, or letters only a millimetre (1/25 inch) high. 
Meaning of the word marked with <WSD> tags:  The size of type between agate and minion, standardized as 6-point.