RADIATION IN THE SOLAR SYSTEM." 



By Prof. J. II. PoYNTiNG, F. K. S. 



I ]ir()p()so to discuss this aftci'iioou coi'taiii efforts of tlio (Micrjify 

 wliich is coiitiniioiisly j)ouriii<j: out fi'oai the sun on all sides with the 

 speed of iijjht, the ener<jy which we call sunliofht when we enjoy the 

 brilliance of a cloudless sky. which we call heat when we l)ask in it.s 

 warmth, the stream of radiation which supports all life on our 

 i>"lobe and is the sourc(> of all our eneroy. 



As we all know, this ceaseless stream of eneroy is a form of wave 

 motion. If we pass a l)eam of sunlio-ht or its efpiivalent, the beam 

 from an electric arc, through a })rism, the disturbance is analyzed 

 into a spectrum of colors, each color of a different wave length, the 

 length of wave changing as we go down the spectrum from, say, one 

 thirty-thousandth of an inch in the red to one eighty-thousandth of 

 an inch in the blue or violet. 



But this visible spectrum is merely the part of the stream of radia- 

 tion which affects the eye. Beyond the violet are the still shorter 

 waves which affect a jihotographic plate or a fluorescent screen, and 

 will pass through certain substances opaque to ordinary light. Here, 

 for instance, is a filter devised l)y Bi'ofessor Wood which stops visi- 

 ble raj's, but allows the shortei- invisible waves to pass and excite 

 the fluorescence of a })latinocyanide screen. 



Again, beyond the red end are still longer waves, which are present 

 in very considerable amount, and can be rendeivd evident by their 

 heating effect. We can easily filter out the visible rays and still 

 leave these long waves in the beam by passing it through a thin sheet 

 of vulcanite. A piece of ])hos))horus ])laced at the focus of these 

 invisible rays is at once fired, or a thermometer quickly I'ises in tem- 

 perature. The waves which have been observed and studied up to 

 the present time range over some nine octaves, from the long waves 

 described to the section yesterday by Professor Rubens, waves of 



oAfternooii nildress <Ielivere<l at the Canibridse meeting of the British Asso- 

 ciation, Aiignst 28. 1004. Reprinted from author's corrected copy. 



185 



