ABSORPTION OF LIGHT, ETC. 



35 



focus afterwards. If we were to take a prism alone, place the 

 prism at a distance from the hole, and in its position of 

 minimum deviation (Fig. C) we should get, if there were two 

 kinds of light only, blue and red, two beams emerging in 

 different directions, the blue (represented in the figure by 

 interrupted lines) being bent round more than the red, 

 and diverging as if they came from two separate points. 

 Now suppose we combine these* two pieces of apparatus 



. B. 



together, placing the prism at a distance from the hole, and 

 the lens near the prism (Fig. D). Then the prism and the 

 lens each fulfils its own office, the prism causes each conical 

 beam of light to be bent round,, but differently, according to 

 the nature of the light, more for the blue than the red ; the 

 lens alone collects each of these conical beams, and brings it 

 ngain to a focus, and so this figure represents what will take 

 place. 



D 2 



