276 FERGUSON'S LECTURES. 



LECT. All bodies appear of that colour, whose rays they 

 v^v^, reflect most ; as a body appears red when it reflects 



most of the red-making rays, and absorbs the rest. 

 Transna- Ay * mui o colours that are quite transparent 

 lours be- D y themselves, become opaque when put together. Thus, 



come if water or spirits of wine be tinged red, and put in 



opaque if . 



put to- a phial, every object seen through it will appear red ; 



gether. because it lets only the red rays pass through it, and 

 stops all the rest. If water or spirits be tinged blue, 

 and put in a phial, all objects seen through it will ap 

 pear blue, because it transmits only the blue rays, and 

 stops all the rest. But if these two phials are held close 

 together, so that both of them may be between the eye 

 and object, the object will no more be seen through 

 them than through a plate of metal ; for whatever rays 

 are transmitted through the fluid in the phial next the 

 object, are stopped by that in the phial next the eye. 

 In this experiment, the phials ought not to be round, 

 but square ; because nothing but the light itself can be 

 seen through a round transparent body, at any dis- 

 tance. 



As the rays of light * suffer different degrees of re- 

 fraction by passing obliquely through a prism, or 

 through a convex glass, and are thereby separated 

 into all the seven original or primary colours ; so they 

 also suffer different degrees of refraction by passing 

 through drops of falling rain ; and then, being reflected 

 towards the eye, from the sides of these drops which 

 are farthest from the eye, and again refracted by pass- 

 ing out of these drops into the air, in which refracted 

 directions they come to the eye ; they make all the co- 

 lours to appear in the form of a fine arch in the heavens, 

 which is called the rain-bow. 



There are always two rain-bows seen together, the 

 interior of which is formed by the rays a b, which fall- 

 ing upon the upper part b, of the drop c d b, are re- 



