1 70 THE HUMAN BODY. 



Short-sightedness, even when slight, is an infirmity from 

 which its subjects suffer all their lives, and it may be aggra- 

 vated by the use of too strong glasses, and, as we have 

 already stated, by the use of the microscope. Long-sighted- 

 ness, on the contrary, does not make itself felt much before 

 the age of forty, and then only in persons whose sight is 

 good. It is, as the name presbyopia indicates, a mark of 

 age, and a little philosophy enables us to resign ourselves to 

 it, and wear the glasses which were useless in youth. 



Achromatism. In ordinary vision objects appear to us in 

 their natural colours distinctly defined, and not surrounded 

 with the iris-like fringe which results from the decomposition 



Fig. 40. 



<7, a. Globes of the eye. 

 bb, b' b' . Objects placed in front of or beyond the point of convergence of 



tJie two axes, 

 c. Point of convergence. 



of light. It would seem therefore that the eye is achromatic. 

 But the experiments of Arago, Frauenhofer, and other scien- 

 tific men, prove that it does not absolutely possess this pro- 

 perty, though it is only when placed under abnormal condi- 

 tions that we discover this. If, for example, we look at an 

 object, and adapt the eye to an imaginary point, either in 

 front of or beyond it, the image both becomes indistinct, 

 and its edges become rainbow-like. If a body is placed 

 near the cornea, in such a way as to cover a portion of the 

 pupil, the same effect is produced. 



Single or double vision with two eyes. Although a separate 

 image is produced in each eye when we look at an object, 

 the object appears single under normal conditions of the 



