IN NATURAL PHILOSOPHY. 39 



34. Why can we see further with a telescope than with the 

 naked eye ? 



Because it furnishes us more light with which to see a dis- 

 tant object. 



35. Why is not snow transparent, like ice? 

 Because it is unhomogeneous. See problem 18. 



36. Are there rays in the sunbeam which we cannot see f 

 We cannot see the heat or the chemical rays. 



37. ( i ) Make two marks on a sheet of white paper, at a 

 distance of about three inches from each other. Then closing 

 one eye and looking steadily at one mark (thoiigh we can see 

 both), move the paper toward the eye. A point will be 

 reached where the eye can perceive only one of the marks / on 

 coming nearer, both will be seen again. 



38. (2) Prick with a pin, through a card-board, two holes 

 :loser together than the diameter of the pupil of the eye. Hold- 

 ing the card pretty near the eyes, look through these holes at 

 the head of a pin. There will seem to be two pin-heads. 



39. (3) Press the finger on one eyeball and we shall see 

 objects double. 



Since an impression is made on the retina of each eye, it 

 would seem that we ought always to see objects double. The 

 nerves from both eyes are so joined, however, before they 

 reach the brain, that this effect is avoided. If, now, we cause 

 the image on the retina to be made on parts of the eye which 

 do not correspond to each other, we shall obtain a double image. 



40. Why is a rainbow in the morning a sign of foul, and 

 in the evening of fair weather? 



In the morning it indicates a formation of clouds when the 

 temperature is rising, and therefore shows a determination to 

 moisture. In the evening it indicates a clearing away when 

 the temperature is falling, and hence shows a determination 

 to dryness. 



41. Why is a red, lowering sky in the morning a sign, of 

 rain, and a brilliant red sky at night, of fair weather f 



