Lesson xvi.] REFRACTION. 83 



at the point of incidence; but if it be a rarer me- 

 dium, in their bending they recede from the per- 

 pendicular. 



We observe in nature many effects of refraction : 

 for instance A staff, one part of .which is immers- 

 ed in water, and the other in air, appears broken ; 

 and that part which is. in water, appears higher 

 than it really is. Again Take a tub, whose sides 

 are straight, and in the middle of the bottom lay 

 a shilling, or any other visible object, fixed so 

 that it cannot move, then go so far from the tub 

 that you can but just see the object, and stand 

 there till another person more than half fills the 

 tub with water, and the shilling will then appear 

 as though it were removed some distance farther 

 from you : this experiment may be varied several 

 ways. . t 



Another remarkable instance of refraction is 

 the following : A person standing by the side of 

 the river Thames at Greenwich, when it is high 

 water there, may see the cattle grazing on the 

 marshy meadow on the other side of the river 

 called the Isle of Dogs ; but when it is low water 

 there, he cannot see any thing of them, they 

 being hid from his view by the bank on the other 

 side the river. This curious effect is probabjy 

 owing to the moist and dense vapours just above 

 the surface of the water, being raised higher pr 

 lifted up with the surface of the water at the tim 

 of high tide; the. rays ..passing through these 

 tapqurs are so much refracted as to render those 



things 



