Chap. 4.] Bottom of a River feems nearer than it is. 203 



traction, fmce it is found by univerfal experience, that 

 the attraction of all bodies is in proportion to their 

 denfities. 



In pafiing from a denfe.into a rare medium, how- 

 ever, there is a certain degree of obliquity at which 

 the refraction is changed into reflection. In other 

 words, a ray of light will not pafs out of a denfe into 

 a rare medium, if the angle of incidence exceed: a cer- 

 tain limit, but will be reflected back. Thus a ray of 

 light will not pafs out of glafs into air, if the angle of 

 incidence exceeds 40 1 1, or out of glafs into water, 

 if the angle of incidence exceeds 59 20. 



As the rays of light, in pafling from a denfe medium 

 to a rarer, are refracted from the perpendicular, in fad! 

 are bent or inclined towards the eye of the fpectator, who 

 looks at an object in the denfer medium while Handing 

 at its fide, the reafon will be clear why the bottom of a 

 river appears to us nearer than it really is * ; and why 

 an oar, partly in and partly out of the water, feems 

 broken. Let Qj?; a (fig. 3.) reprefent an oar^ the part 

 m QJbeing out of, and the part ma being in the water 3 

 the rays diverging from a will appear to diverge from 

 b nearer to the furface of the water, every point in m a 

 will be found nearer to the furface than its real place, 

 and the part ma will appear to make an angle with 

 the part Qni. On this account alfo, a fifli in the wa- 

 ter appears much nearer the furface than it actually is; 

 and a fkilful markiman, in fbooting at it, will aim con- 

 fiderably below the place which it feems to occupy. t 



On the fame principle a common experiment is ex- 

 plained. Put a Ihilling into a bafon, and walk back 

 from it till the fhilling is juft-obfcured by the fide of 



* If the fpeftator ftands on a bank, juft about the level of the 

 water, it is about one-third deeper than it appears. 



the 



