502 THE HUMAN BODY. 



matters in the demeanor or expression of those with 

 whom they converse, which, being noticed by those of 

 quicker sight, might induce feelings of distrust or annoy- 

 ance." 



In old age the lens loses some of its elasticity and be- 

 comes more rigid. This leads to the long-sightedness of 

 old people, known as presbyopia. The stiffer lens does not 

 become as convex as it did in early life, when the ciliary 

 muscle contracts and the suspensory ligament is relaxed. 

 A special effort of accommodation is therefore needed in 

 order to adapt the eye to see near objects distinctly; and 

 convex glasses are required. 



In all forms of deficient accommodation too strong glasses 

 will injure the eyes irreparably, increasing the defects 

 they are intended to relieve. Skilled advice should there- 

 fore be invariably obtained in their selection, except per- 

 haps in the long-sightedness of old age when the sufferer 

 may tolerably safely select for himself any glasses that 

 allow him to read easily a book about 30 centimeters (12 

 inches) from the eye. As age advances stronger lenses 

 must of course be obtained. 



Optical Defects of the Eye. The eye, though it an- 

 swers admirably as a physiological instrument, is by no 

 means perfect optically; not nearly so good, for example, 

 as a good microscope objective. The main defects in it are 

 due to 



1. Chromatic Aberration. As already pointed out the 

 rays at the violet end of the solar spectrum are more re- 

 frangible than those at the red end. Hence they are 

 brought to a focus sooner. The light emanating from a 

 point on a white object does not, therefore, all meet in one 

 point on the retina; but the violet rays come to a focus first, 

 then the indigo, and so on to the red, farthest back of all. 

 If the eye is accommodated so as to bring to a focus on the 

 retina parallel red rays, then violet rays from the same 

 source will meet half a millimeter in front of it, and cross- 

 ing and diverging there make a little violet circle of diffu- 

 sion around the red point on the retina. In optical instru- 



