68 REFRACTION OF LIGHT, 



perpendicular. If you put a piece of money into an empty 

 basin, and stand at such a distance that it may not be visi- 

 ble ; then let another person pour water into the basin, and 

 the money will be seen ; for the rays of light, in passing 

 from a denser into a rarer medium, are bent froin the per- 

 pendicular, and thus are directed to your eye. The follow- 

 ing, therefore, may be established as a sort of axiom in op- 

 tics : we see every thing in the direction of that line in which 

 the rays approach us last. If you place a candle before a 

 looking-glass, and stand before it, the image of the candle 

 appears behind it ; but if another looking-glass be so placed 

 as to receive the reflected rays of the candle, and you stand 

 before this second glass, the candle will appear behind that; 

 because the mind imagines every object to be in the direc- 

 tion from which the rays come to the eye last. Hence, when 

 the rays of light coming from the celestial bodies, arrive at 

 Qur atmosphere, they are bent downward ; and those bodies 

 appear, when in the horizon, higher than they are. The 

 effect of this refraction is about six minutes of time, but the 

 higher they rise, the less are the rays refracted ; and when 

 they are in the zenith, they suffer no refraction. The sun 

 is visible about three minutes before he rises, and about the 

 same time after he sets ; making in the course of a year 

 about a day and a half Twilight is occasioned partly by 

 refraction, but chiefly by reflection of the sun's rays by the 

 atmosphere, and it lasts till tiic oJn is eighteen degrees be- 

 low the horizon. Were there no atmosphere to reflect and 

 refract the sun's rays, only that part of the heavens would 

 be luminous in which the sun is placed ; and if we could 

 live without air, and should turn our backs to the sun, the 

 whole heavens would appear as dark as in the night. In 

 this case also, a sudden transition from the brightest sun- 

 shine to dark night would immediately take place upon the 

 setting of the sun. 



Questions. — 1, What is said of optics ? 2. In what manner is 

 light projected from luminous bodies ? 3. What is still a disputed 

 point, and what is said of it ? 4. How are rays of light reflected ? 5. 

 How is it shown that the angle of refleetionis equal to the angle of 

 incidence ? 6. What is raeant by the refraction of rays of light ? 7. 

 How are they refracted in passing from a rarer into a denser medium ? 

 8. From a denser into a rarer ? 9. What is the example for illustration ? 

 10. What may be established as a sort of axiom in optics ? 11. Give 

 the illustration. 12. What is the effect of rays of light, coming from 



