OF OPTICS. 



229 



excite in our minds the idea of light. And when they LECT. 

 fall upon bodies, and are thereby reflected to our eyes, v^-v-^. 

 they excite in us the ideas of these bodies. And as every 

 point of a visible body reflects the rays of light in all 

 manner of directions, every point will be visible in every 

 part to which the light is reflected from it. Thus the 

 object A C B is vi- 

 sible to an eye in 

 any part where the 

 rays A a, Ab, Ac, 

 Ad, Ae, Ba, Bb, 

 Be, Ed, Be, and 

 Ca, Cb, Cc, Cd, 

 C e, come. Here 

 we have shewn the 

 rays as if they were 



only reflected from the ends A and B, and from the mid- 

 dle point C of the object ; every other point being sup- 

 posed to reflect rays in the same manner. So that wher- R fl ectc a 

 ever a spectator is placed with regard to the body, every light 

 point of that part of the surface which is towards him 

 will be visible, when no intervening object stops the 

 passage of the light. 



Since no object can be seen through the bore of a 

 bended pipe, it is evident that the rays of light move in 

 straight lines, whilst there is nothing to refract or turn 

 them out of their rectilineal course. 



Whilst the rays of light continue in any medium** of 

 a uniform density, they are straight; but w^hen they 

 pass obliquely out of one medium into another, which is 

 either more dense or more rare, they are refracted 

 towards the denser medium : and this refraction is more 

 or less, as the rays fall more or less obliquely on the 

 refracting surface which divides the mediums. 



Note 63. Any thing through which the rays of .light can pass, is 

 called a medium ; as air, water, glass, diamond, or even a vacuum. 

 \ote by the Author. 



