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As some rays of light are less than others, so they 

 are more refrangible : these which are most refrangible, 

 constitute violet colour that is, the smallest rays excite 

 the most languid colour. Those which are largest, and 

 so least refrangible, constitute red, the most vivid colour. 

 The other rays excite intermediate sensations, according 

 to their respective size and relrangibility. 



Bodies reflect, instead of transmitting light, that is 

 are opake, riot transparent, not for want of pores; but 

 either because of the unequal density of their parts or 

 the magnitude of their pores. Either their pores are 

 empty, or they are filled with matter of a different kind, 

 whereby the rays are variously refracted and reflected 

 till they are quite absorbed. 



Hence paper and wood are opake, while glass is 

 transparent ; for in the confines of parts alike in den- 

 sity (such as those of glass and water) there arises no 

 refraction or reflection, by reason of the equal attrac- 

 tion every way ; so that the rays which enter the 

 first surface pass strait through the body. But in the 

 parts of wood and paper, which are unequal in density, 

 and contain much air HI their large pores, the refractions 

 and reflections are very great, so that the rays cannot 

 pass through them, but are bandied about till they are 

 extinguished. 



Hence opake bodies become transparent when their 

 pores are filled with a substance of equal density, as 

 paper dipt in water or oil : and, on the contrary, trans- 

 parent bodies, by emptying their pores or separating 

 their part?, become opuke. Thus salts and wet paper 

 become opake by drying, glass by pulverizing. Yea, 

 water itself, it beat into froth, loses its transparency. 



That lig' t is corporeal cannot now be doubted, having 

 been proved by a thousand experiments. By reflection 

 and refraction it may he turned more or less out of its 

 Hay, acroiding to the different densities of the reflecting 

 or retracting medium. Its rays in their progressive 

 motion may be intercepted by the interposal of any opake 

 object : and when this is removed, they proceed again 



