350 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 3 



ray separately and determine the different refraction for each one. 

 He could reunite the seven rays by a new refraction, thus reconsti- 

 tuting one ray of white light. 



These seven rays of light made visible to us when separated by a 

 prism from the white light of a sunbeam are not the only rays that 

 come from the sun ; there are other rays that are completely invisible 

 to our eyes. Beyond the red rays, making no impression on our 

 eyes, are the so-called " heat waves " which have longer wave lengths 

 than the red waves, and therefore are known as infrared. Wlien the 

 sun is at an angle of 45°, or midway to the zenith, 60 percent of the 

 rays that may reach the earth at sea level in the direct beam are 

 heat waves, and under 40 percent may be light rays. 



ELECTROMAGNETIC RADIATION 



COSMIC RAYS 



CENTIMETERS 



4220 3S0C ANGSTROMS 



'■^n ORANGE YELLOW GREEN BLUE VIOLET 



Figure 1. — Diagram showing the position of the ultraviolet region In the great electro- 

 magnetic spectrum. Data from W. Edwards Demlng and P. G. Cottrell. Arranged by 

 Earl S. Johnston. 



At the other end of the spectrum, beyond the violet rays in sun- 

 light, there is still another set of rays, invisible to our eyes but of 

 vital importance to life on the earth. These rays are shorter in 

 wave length than the violet, so they are called the ultraviolet rays. 

 (See fig. 1.) Only a very small quantity of them — less than 1 per- 

 cent — may be present in the sunlight that reaches the surface of the 

 earth. Outside the earth's atmosphere there is a much larger per- 

 centage of them, but the ozone formed from oxygen in the upper 

 layers of the atmosphere by the action of these ultraviolet rays serves 

 as a ray filter that protects the life on the surface of the earth from 

 these shorter rays which have been proved to be very destructive to 

 tender tissues. 



Evidence of the lethal action of these ultraviolet rays was re- 

 vealed for the first time to the scientific world when Downes and 

 Blunt (1877) reported that the short-wave-length ultraviolet rays 



