44 THE REASON WHY . 



There are more tilings in heaven and earth, Horatio, 

 Thau are dreamed of in your philosophy.&quot; SUAKSPERE. 



126. Why are some persons short-sighted? 



Because the rays of light are brought to a focus before they reach 



the retina, and consequently 

 produce an indistinct picture 

 on the retina. This will be 

 more clearly illustrated by the 

 accompanying engraving. 



127. The remote causes of this defect of the eye may be various ; as too great 

 density of the humours too great a convexity either of the cornea or the lens and 

 from the space between the retina and the lens being too short. Where it depends 

 on too much convexity, as age approaches, it may disappear ; but where too great 

 density is the cause, age is apt rather to aggravate than to relieve it. For while in 

 the young the convexity both of the cornea and the lens is greater than in advanced 

 life, the less density of the humours counterbalances it ; and again as life advances, 

 the more scanty supply of humours, along with the diminishing convexity, are 

 neutralised by the increasing density ; therefore, if these keep pace with each other 

 the eye remains to a very protracted period of life without the necessity for 

 Artificial assistance. 



128. W]iy do the eyes sometimes become &quot;bloodshot?&quot; 



Because when the eye is healthy its blood-vessels are so small 

 that only the colourless portion of the blood finds its way through 

 them. But when the eye is inflamed, the coats of these vessels be 

 coming weaker, they are readily distended by the impulse of the 

 blood against them ; they then become enlarged, the red globules 

 find their way into them, and they are rendered red and visible to 

 the naked eye. 



129. Why, when the sight is impaired in old age, are persons 

 compelled to wear spectacles, or to hold objects at a distance from 

 them in order to distinguish them perfectly ? 



Because the diminished refractive power of the organ is accom 

 panied by a divergence of the rays of light which tends to confuse 

 and obscure the sight. By holding objects at a distance the 

 divergence becomes diminished, and the same effect is produced 

 by convex glasses. 



