OF OPTICS. 



f!31 



B. But if the vessel be filled with water, the point a LECT. 

 will be seen from e ; and will appear as at d, elevated in ^-N,-^ 

 the direction of the ray e B." 



The time of sun-rising or setting, supposing its rays The daTS 

 suffered no refraction, is easily found by calculation. ar e made 

 But observation proves that the sun rises sooner, and the re- 



sets later every day than the calculated time ; the 

 reason of which is plain from what was said immediately sun's rays 

 above. For, though the sun's rays do not come part of 

 the way to us through water, yet they do through the 

 air or atmosphere, which being a grosser medium than 

 the free space between the sun and the top of the at- 

 mosphere, the rays by entering obliquely into the at- 

 mosphere, are there refracted, and thence bent down to 

 the earth. And although there are many places of the 

 earth to which the sun is vertical at noon, and conse- 

 quently his rays can suffer no refraction at that time, 

 because they come perpendicularly through the atmo- 

 sphere : yet there is no place to which the sun's 

 rays do not fall obliquely on the top of the atmo- 

 sphere, at his rising and setting; and consequently, no 

 clear day in which the sun will not be visible before he 

 rises in the horizon, and after he sets in it ; and the 

 longer, or shorter, as the atmosphere is more or less 

 replete with vapours. For, let A B C be part of the 



Note 64. Hence a piece of money lying at e, in the bottom of an 

 empty vessel, cannot be seen by an eye at a, because the edge of the 

 vessel intervenes ; but let the vessel be filled with water, and the 

 ray e a being then refracted at B, will strike the eye at a, and so ren- 

 der the money visible, which will appear as if it were raised up to/ 

 in the line a B /. 



