152 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 8 



We are all familiar with the fact that if sunlight is split into its 

 component colors by means of a spectroscope we can see a large variety 

 of the radiations represented by the various parts or colors of the solar 

 spectrum. The visible range to which the eye responds represents 

 frequencies extending from 400 million million cycles per second to a 

 frequency just about double this, or 800 million million cycles per 

 second. The sensation of the higher of these two frequencies is that of 

 violet light, and the sensation produced by the 400 million million cycle 

 frequency is that of deep red light. In between these two extremes 

 of the spectrum fall the intervening colors. But outside this so-called 

 visible range to which the eye responds there is a vast scale of radia- 

 tions both beyond the red end of the spectrum which we call the 

 infrared and far down below the violet which we call the ultraviolet. 



By means of the photographic plate, we can extend the map of the 

 spectrum in either direction. Far out beyond the red end are heat 

 radiations from the sun that may be measured with the thermopile or 

 the bolometer. It was during the days of Dr. Samuel Pierpont 

 Langley, a predecessor of Dr. Abbot as Secretary of the Smithsonian 

 Institution, that a very complete and careful survey of the radiations 

 from the sun was carried out, extending into the infrared. Today 

 much research is being done in measuring the extremely short waves, 

 or high-frequency radiations out beyond the violet, for the ultraviolet 

 is coming to have increasing importance not only from the point of 

 view of health but from the point of view of the radio engineer. 



It is undoubtedly the impact of these very short waves or high- 

 frequency radiations upon the top of the atmosphere that is responsi- 

 ble for its ionization that makes possible all our radio communication. 

 Fortunately for us, most of the ultraviolet light is stopped in the upper 

 atmosphere owing to the presence of a small amount of ozone which 

 acts as an absorbing screen to these high-frequency radiations. If the 

 ozone in the upper air were reduced to standard atmospheric condi- 

 tions at the earth's surface, it would form a layer scarcely more than 

 2 millimeters thick. Through the 2-millimeter layer a sufficient amount 

 of ultraviolet still seeps through to the earth's surface to produce the 

 necessary vitamins for our well-being. Were this ozone layer absent, 

 on the other hand, it would be quite impossible for us to survive under 

 the extremely short wave-length radiations that would penetrate to 

 the earth's surface. 



Thus, we see it is the combination of both the sun and our atmos- 

 phere that makes life on the earth possible. The sun not only 

 radiates health-giving sunshine but it also radiates death-dealing rays. 

 Were it not for the protecting shield of the earth's atmosphere, the 

 sun would be the death of all of us. The atmosphere, then, provides 

 us, on the one hand, with oxygen for maintaining life, and on the other 

 hand, shields us from the highly penetrating rays from the sun that 



