S&ORT-SIGHTEDNE9S. 161 



to a focus sooner, or what is the same thing, the image 

 would be formed a little in front of the retina, instead 

 of directly upon it. 



Dr. B. And what means would you take to remedy 

 it^ provided such an affection should take place ? 



Emily. I am sure Dr. B., I cannot tell any more 

 than the man in the moon. I suppose physicians make 

 use of plasters and drugs, whose very names are enough 

 to terrify one. 



Dr. B. The means I refer to, act on optical prin- 

 ciples, and are what you see made use of every day. 

 This change of the humors takes place in short sighted 

 people, and is perfectly well remedied by using concave 

 spectacles. 



Emily. These, by increasing the divergency of the 

 rays, will counteract the undue convergency produced 

 by the change in the density of the humors. 



Dr. B. If on the contrary, the humors are not suffi- 

 ciently dense, and therefore do not converge the rays 

 enough, the focus will be at a point beyond the retina, 

 and the image will be very indistinct. This affection is 

 peculiar to old people. 



Emily. And is corrected by using convex spectacles. 

 I never imagined before, that the defect of vision in old 

 and young people, proceeded from precisely opposite 

 causes the one from too little, and the other, from too 

 great convergency of the rays. But what are the caus- 

 es of these defects? 



Dr. B. The former, or that of old people, seems 

 to be the natural consequence of old age, though some- 

 times it is undoubtedly brought on by other causes 

 that of being accustomed to look a great, deal at very 

 distant objects. The short sightedness of young people 

 is chiefly produced by looking at objects placed too near 

 the eye. In children, it frequently is a consequence of 

 their books being held too near. I knew a respectable 

 school where short sightedness was a very common af- 

 fection among the students, by reason of the desks beine; 

 14* 



