INTRODUCTION TO OPTICS. 



Ixxxvii 



appear to be situated in the direction of the rays which enter the eye, the 

 shilling 1 will be seen in the direction of the refracted ray at B. 



The manner in which an oar appears bent in water is a similar effect 

 of refraction. The line drawn from the point A to the shilling in the 

 last figure may now be conceived to represent an oar plunged in water; 

 the lowest point being* represented in the figure by the little circle, will, 

 as before, have an image of itself formed apparently above it, as at B. 

 In like manner every point of the oar below the surface of the water will 

 have an image of itself formed above it at some point in the dotted line 

 in the figure, and the whole dotted line will represent the whole image 

 of that part of the oar which is immersed. The part of the oar above the 

 water, extending to A, is seen in its natural position ; that below the 

 water is seen as if reaching along the dotted line to B ; the oar, there- 

 fore, appears bent or broken at the surface of the water. The fact of the 

 formation of an image above the true place of the body does not depend 

 on the situation of the eye. The representation of the eye in the figure 

 therefore, which was introduced in the former paragraph, to explain the 

 cause of a single ray, is not necessary or particularly applicable to the 

 present subject. 1 



When we see the bottom of a clear stream, the rays which it reflects, 

 being refracted in their passage from the water into the air, will make the 

 bottom appear more elevated than it really is, and the water will conse- 

 quently appear more shallow. Accidents have frequently been occasioned 

 by this circumstance ; and boys who are in the habit of bathing should be 

 cautioned not to trust to the apparent shallowness of water, as it will 

 always prove deeper than it appears. 



The refraction of light prevents our seeing the heavenly bodies in 

 their real situation. The light they send to us being refracted in 

 passing into the atmosphere, we see the sun and stars in the direction 



Fig. 21. 



E 



of the refracted ray, as described in Jig. 21, where the dotted line 

 represents the extent of the atmosphere, above a portion of the earth, 

 E B E. A ray of light coming from the sun, S, falls obliquely on it at 

 A, and is refracted to B ; then, since we see the object in the direction 

 of the refracted ray, a spectator at B will see an image of the sun at C, 

 instead of the real object at S. If the sun were immediately over our 

 heads, its rays falling perpendicularly on the atmosphere would not be 

 refracted, and we should then see it in its true situation. To the inha- 

 bitants of the torrid zone, where the sun is sometimes vertical, its rays 

 are then not refracted. There is, however, another obstacle to seeing the 

 heavenly bodies in their true situation, which affects them in the torrid 

 zone as well as elsewhere. Light is about eight minutes and a half in 

 its passage from the sun to the earth ; therefore, when the rays reach 



