2l8 ANSWERS TO PRACTICAL QUESTIONS 



34. Why is rough glass translucent while smooth glass is 

 transparent ? 



. The minute protuberances scatter the rays of light 

 and do not allow them to pass freely to the eye of the 

 observer. 



35-42. (See Key, pp. 38, 39 ; Questions 28-35.) 



43. Are there rays in the sunbeam which we cannot 

 perceive with the eye ? 



(See Physics, pp. 163, 164.) 



The calorific and actinic rays are invisible. 



44. Why, when we press the finger on one eyeball, do we 

 see objects double ? 



" Each retina possesses regions of symmetry with the 

 other, and on this fact singleness of vision depends; 

 each point of the outer portion of the retina of the right 

 eye has its point of symmetry in an inner portion of the 

 left, and when from a distant object rays fall on these 

 symmetrical points, that object will be seen single ; but 

 if, by the pressure of the finger or otherwise, we compel 

 the image in one of the eyes to fall upon another 

 and non-symmetrical point, the object at once becomes 

 double." 



(See Draper's Human Physiology^ p. 395.) 



45. Why does a distant light, in (lie night, seem like a 

 star? 



The light radiating in every direction produces the 

 star-like effect, and we cannot see the surrounding objects 

 by which to correct the impression. Hence one often 

 mistakes a fire on a distant hill for a star rising. 



