FIG. 131. JOSEPH BLACK. 



CHAPTER XII. 



PHYSICS OF THE EIGHTEENTH CENTURY. 



THE property of a lens to form an image depends upon its power 

 of refracting the rays of light ; but as the refrangibility of the 

 rays of light varies according to their colour, differently coloured rays 

 of light will be differently refracted by the same lens, and will be 

 brought to foci at different distances from the lens. Newton's experi- 

 ment (page 218) showed that ordinary light by the different refrangi- 

 bilities of its coloured constituents is decomposed in passing through 

 a prism, and it will have already been observed that a lens acts by the 

 same laws as the prism (page 157). Hence, with ordinary light, a 

 common convex lens does not form a single image, but an indefinite 

 series of images. The violet rays, as the most refrangible, are brought 

 to a focus nearest the lens, while the red rays have their focus at a 

 greater distance, and at intermediate positions are images due to the 



