410 ^ LECTURE XXXV. 



nature of the medium is changed, the path of Ijght dcA'iates from a straight 

 line: thus, the apparent places of the sun and stars are changed by the effect of 

 the atmosphere, because the light, by which we judge of their situations, is 

 deflected, in its passage out of the empty space beyond the atmosphere, 

 first into the rarer and then into the denser air. In the same manner, when 

 we view a distant object over a fire or a chimney, it appears to dance and 

 quiver, because the rays of light, by which it is seen, are perpetually thrown 

 into new situations, by the different changes of the density of the air in con- 

 sequence of the action of heat. 



"When rays of light arrive at a surface, which is the boundary of two me- 

 diums not homogeneous, they continue their progress without deviating 

 from those planes, in which their former paths lay, and which are perpen- 

 dicular to the surface of the mediums; but they no longer retain the same 

 direction, a part of them, and sometimes nearly the whole, is reflected back 

 from the" surface, while the remaining part is transmitted and refracted, or 

 bent. The name refraction is derived from the distortion which it occasions 

 in the appearance of an object viewed in part only by refracted light: thus 

 an oar, partially immersed in water, appears to be bent, on account of the re- 

 fraction of the light by which its lower part is seen, in its passage out of the 

 water into the air. 



There is no instance of an abrupt change of the density of a medium, 

 without a partial reflection of the light, passing either into the denser or into 

 the rarer medium; and the more obliquely the light falls on the 

 surface, the greater, in general, is the reflected portion. No body 

 is so black as to reflect no light at all, and to be perfectly invisible 

 in a strong light; although at the surface separating two very rare bodies, 

 as two kinds of gas, the reflection is too faint to be perceptible; but in this 

 case the separation is seldom perfectly abrupt. 



The angles of incidence and reflection are the angles made by a ray of 

 light, before and after its reflection, with a line perpendicular to the reflecting 

 surface; and these angles are always equal to each other; consequently 

 the inclination of the rays to the surface remains also the same. The 

 quantity of light reflected, when other circumstances are equal, appears to 



