RADIATION IN THE SOLAR SYSTEM." 



Bv Prof. J. II. I'oYNTiNG. F. K. 8. 



I ])i'())^ose to discuss this aftcnioon ccilain etrocts of the cnorffy 

 which is continuously pourino- out from the sun on all sides with the 

 speed of lio^ht, the energy which we call sunlio-ht when w^e enjoy the 

 hrilliance of a cloiulless sky. which we call heat when we bask in it.s 

 waj'nith, the stream of radiation which sup|)orts all life on our 

 g'lobe and is tlie source of all our ener<i'v. 



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

 motion. If we pass a bcnim of sunlight oi- its equivalent, the beam 

 from an electric arc, through a jirisni, the disturbance is analyzed 

 into a spectrum of colors, each color of a dilferent Avave length, the 

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

 thirty-thousandth of an incli in the re(l to one eighty-thousandth of 

 an inch in the blue or violet. 



But this visible si)ectrum is merely tlie part of the stream of radia- 

 tion which aifects the eye. l)eyond the violet are the still shorter 

 waves which atl'ect a ])hotographic ])late or a fluorescent screen, and 

 will })ass through certain substances opa<|ue lo ordinary light. Here, 

 for instance, is a filtei' devised by Professor Wood which stoj^s visi- 

 ble rays, but alloAvs the shorter in\isible 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 considei'able amount, and can be rendeivd evident by their 

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

 leave these long waves in the beam by })assing it through a thin sheet 

 of vulcanite. A piece of phosphoi'us ])laced at the focus of these 

 invisible rays is at once fired, or a tluu'inometer quickly rises in tem- 

 ])erature. 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 RubcMis, waves of 



^Afternoon address delivered at the Cambridge meeting of the British Asso- 

 ciation, Angiist 2:i. 1004. Reprinted from author's corrected coi)y. 



185 



