Experimental Philosophy. [Lecture 10. 



and they act like a straight tube, which allows 

 them a free passage ; and those bodies are opake 

 whose pores are not straight, and which there- 

 fore intercept the rays, like ,the bent tube already 

 mentioned. 



If the rays of light proceed in straight lines, it 

 is obvious that they must be sent from every 

 visible object in all directions. It is however 

 only by those rays which enter the pupil of our 

 eye that they are rendered visible to us ; but, 

 being sent in all directions, it is evident that 

 some rays from every part must reach the eye- 

 Thus the object ABC (pi. XI. fig. 46) is rendered 

 visible to an eye in any part, where the rays Aa, 

 Ab, Ac, Ad, Ae, Ba, Bb, Be, Bd, Be, Ca, Cb, 

 Cc, Cd, Ce, can come ; and these affect our 

 sight with the sense of different colours and 

 shades, according to the properties of the body 

 from which the light is reflected, as will be ex- 

 plained when we come to treat of colours. 



Of the refraction and refaction of light I shall 

 hereafter treat more at large ; but, ip the mean 

 time, it will greatly facilitate the study of optics, 

 if you will carefully peruse, and still more if you 

 will commit to memory, the following principles 

 and definitions. 



1. Light is a substance, the particles of which 

 are extremely minute, which, by striking on our 

 visual organs, gives us the sensation of seeing. 



2. The particles of light are emitted from what 

 are called luminous bodies, such as the sun, a 



