30 THE ELEMENTS OF VEGETABLE HISTOLOGY 



the edges; and the resulting differences in focal points 

 give rise to blurred images. Ordinary or white light 

 is composed of rays of different lengths, each of which 

 produces a specific color. A glass prism, of which 

 a lens is but a modification, has the power of separat- 

 ing white light into its component rays which are 

 of different lengths, and consequently of different 

 colors. The play of colors resulting from the passage 

 of light through a prism or lens is termed the spectrum 



FIG. 14. Chromatic Aberration. 



The lens acts as a. prism and causes dispersion of white light into rays of different 

 colors. 1. Ultraviolet focal point. 2. Violet focal point. 3. Blue focal point. 

 4. Green focal point. 5. Yellow focal point. 6. Orange focal point. 7. Red focal 

 point. 



and will show the colors in the following order: red, 

 orange, yellow, green, blue and violet. The red rays 

 are the longest and the violet rays are the shortest 

 (Fig. 14). Owing to the difference in wave lengths 

 the red rays will come to a focus at a greater distance 

 from the lens than the violet rays. These differences 

 in focal points give rise to blurred and colored edges 

 around the image and are responsible for the defect 

 termed chromatic aberration. Both of these defects 

 are corrected by using glass of different kinds in the 

 construction of compound lenses and combining dif- 

 ferent types of lenses. Achromatic lenses are those 

 in which chromatic aberration has been corrected. 



