166 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 



ordinary receiver, however, other radio waves exist, and all that is 

 necessary to intercept them is a receiving set suitably designed. 

 Thus we have radio wave lengths ranging from a few meters to 

 many kilometers, all liaving the same physical nature and traveling 

 through space with the same velocity. 



In order to make more definite the idea of the relative magnitude 

 of different wave lengths reference may be made to Figure 1. 

 Here is represented the region of the ordinary radio waves, and, 

 following along the scale, we find a succession of other types of 

 waves, all of which, however, are physically alike, just as all sound 

 waves are alike, though differing in pitch. Here you see the place 

 occupied by the visible waves, here the ultra-violet and the famous 

 cosmic radiation, of which so much is lately heard. This diagram, 

 doubtless familiar to many of you, is shown especially to point out 

 the location of the particular field selected for discussion — beA'^ond 

 the red, but not indefinitely beyond. It also illustrates the fact that 

 in a broad sense we are all exceedingly color blind. 



Radio waves Waves from Infra-red J».-Ultfa-viol8t Border X-rays y-rays ^„ 



elecUic 'Sm "gm |j 



discharges i>~ O 



FiGUEB 1. — The octaves of light from The World of Atoms, by Arthur Haas, 

 English trauslalion by H. S. Uhler. By permission of the D. Van 

 Nostrand Co. 



Having seen the setting of our subject, let us consider a little of 

 its history. All astronomers honor the name of Sir William Herschel, 

 who with his son, Sir John, contributed so much to our knowledge 

 of the universe. His enthusiasm, skill, and insight brought rich 

 returns. He explored the depths of space with an unprejudiced 

 mind, the gift of his Creator, and an excellent telescope, which he 

 made for himself. He sought a way to use his poAverful instru- 

 ment for visual examination of the sun ; how to avoid the disastrous 

 effects of too much heat and light in the eyepiece without sacrificing 

 the power of his instrument. This led Sir William to make the 

 first investigation of what lies beyond the red in the spectrum of 

 sunlight. With a glass prism, three ordinary thermometers, a 

 watch, some boards and paper screens, he carried out a remarkable 

 series of observations, an account of which he published in the year 

 1800 in the Philosophical Transactions of the Royal Society of 

 London under the title "Investigation of the Powers of the Pris- 

 matic Colours to Heat and Illuminate Objects." 



In this paper Herschel gives a most interesting description of his 

 experimental work. He relates in detail how one thermometer was 

 exposed to the infra-red part of the solar spectrum, while the others 



