252 THE HUMAN MECHANISM 



they are not. Gas from an ordinary fishtail burner is one 

 of the poorest lights for reading and sewing. The flicker of 

 gas lights may, however, be largely avoided by the use of 

 mantles. 



3. If the printed matter is not held steady, the effort of 

 accommodation becomes much more difficult. Consequently 

 it is in general a bad thing to read, and especially to read 

 fine or poorly printed matter, on any but the steadiest 

 railroad train. 



4. The use of very fine type should be reduced to a mini- 

 mum. When such printed matter is held at the ordinary 

 distance of eighteen inches from the eye, very accurate 

 accommodation is needed, and this, we have just seen, 

 involves nervous strain ; if it is held closer to the eye (so as 

 to make a larger image on the retina) the lens must be made 

 much more convex to focus it, and this means excessive 

 work on the part of the muscle of accommodation. This is 

 very undesirable, and especially so in youth, since then the 

 tissues of the eye are more plastic, and excessive strain of 

 the muscle of accommodation, pulling as it does on the scle- 

 rotic and the choroid coats, may lead to permanent deforma 

 tion of the curved surfaces. The marked increase of myopia 

 within the past forty or fifty years is generally explained in 

 this way. 



5. Highly calendered paper objectionable. Closely connected 

 with the size of the type is the character of the paper on 

 which it is printed. This should be as dull as possible in 

 order to avoid the confusing effect of a glossy surface. The 

 use of highly calendered paper in many books and serial 

 publications, because such paper lends itself more readily 

 to the reproduction of pictures in half tone, is a sacrifice 

 of hygienic considerations to cheapness. 



14. Visual sensations. We have shown (p. 241) that the 

 sensation of sight does not develop in the eye, but in the 

 brain, as the result of nervous impulses sent thither over 



