210 ANNUAL REPORT SMITHSONIAN INSTITUTION,, 1942 



Within the past decade the susceptibility of microorganisms to ultra- 

 violet radiation has received renewed attention. 



FORMS OF RADIANT ENERGY 



What is ultraviolet radiation and what has this to do with sun- 

 light? The visible spectrum was discovered in 1665. This visible 

 spectrum which was thought to be one band of light, colored white, 

 was found to be in reality a combination of many different colors of 

 light. You know what happens when a beam of clear bright sun- 

 light strikes a suitable prism or if you gaze at the rainbow in the sky 

 where Nature performs the experiment, you observe tiny drops of 

 moisture present in the air acting as prisms breaking the white ray 

 of sunlight into many visible colored rays, red, orange, yellow, green, 

 blue, and violet. In addition to these bands of visible radiation there 

 are other rays emanating from the sun which are invisible to the 

 human eye. Some known as infrared rays are invisible, but this ra- 

 diation is detected by the heat produced. Others spoken of as ultra- 

 violet rays, also are invisible; this form of radiant energy is not de- 

 tected by any of our senses, although some of the effects produced 

 subsequently may become apparent. These three forms of radiation, 

 one visible and the other two invisible, all emanating from the sun, 

 differ in the lengths of their waves. The ultraviolet rays are the 

 shortest; the visible light rays are longer; and the infrared waves are 

 the longest. 



The unit of measurement in microscopy is the micron which is 

 equivalent to l/25,000tli of an inch. Wave lengths of light rays are 

 measured in terms of Angstrom units. An Angstrom unit is 

 l/10,000th of a micron; in 1 inch there are 250 million Angstrom 

 units. The visible light is subdivided into colors of the spectrum 

 by the energy of different wave lengths of an approximate range 

 from the red, 8000 Angstrom units, to the violet, 4000 Angstrom 

 units. To be exact, the longest visible rays, the red, have a length 

 of 7610 Angstrom units; and the shortest visible rays, the violet, 

 have a length of 3970 Angstrom units or approximately l/63,000th 

 of an inch. The infrared rays, which are heat rays, are longer than 

 the red and the ultraviolet rays are shorter than the violet. 



ULTRAVIOLET LIGHT 



The ultraviolet portion of the spectrum consists of wave lengths 

 which extend from a range of 3970 Angstrom units to about 1000 

 Angstrom units. By passing a beam of ultraviolet light through a 

 quartz prism, this can be subdivided approximately into three divi- 

 sions, the far ultraviolet rays (1000 to 2000 Angstrom units), middle 

 ultraviolet (2000 to 3000 Angstrom units), and near ultraviolet (3000 



