MODERN SUN CULT — STURMER 193 



we had better not be clo<;in:itio on this subject which is linked with 

 the finuhiinental conc'ei)ts of eneiiry and matter — concepts wliich have 

 been niodilied quite recently, and may soon be njodified still further. 

 Most physicists now subscribe to the corpuscular theory, in accord- 

 ance with which lii;ht, and the ultra-violet and the heat rays as well, 

 come from the sun as a stream of corpuscles, emitted in a succession 

 of bursts, but the latter so close tojjether, and following one upon 

 the other with such inconceivable speed, that the stream is, as far 

 as we can perceive, continuous and unbroken. 



WAVE LENGTHS 



When, however, solar radiations are to l)e dealt with in their prac- 

 tical aspects, as for instance in i-eixanl to reflection or refraction, it 

 is the other theory of li^ht, the wave theory, that provides the 

 nomenclature. So we continue to speak of wave lengths, and of wave 

 frequencies, just as we do in radio parlance. Indeed, it is generally 

 accepted that certain light waves may be " tuned in," and other light 

 waves refused entry. On this basis we explain the phenomenon of 

 color. But to understand the exi)laiiation we must remember that 

 the light waves of the dilTerent colors of the spectrum differ in 

 length; that the red waves are the longest, and that the waves become 

 shorter and shorter, through the orange, yellow, green, blue, and 

 violet, the latter being less than half the length of the red. 



ULTRA-VIOLET 



It must not be assumed that ultra-violet rays are of a single wave 

 length. On the contrary, they embody a range of wave lengths, just 

 as light rays represent a range. The longest waves of ultra-violet 

 are just a little shorter than the shortest violet waves. They are thus 

 just beyond the violet rays of the spectrum. Hence the term ultra- 

 violet. It is not quite correct, however, to speak of ultra-violet light, 

 for light is visible, while the rays of shorter wave lengths than those 

 of light are invisible. Hence we had better use the term ultra-violet 

 radiation. 



THE Angstrom unit 



We must bear in mind that the waves of light rays are exceedingly 

 mimite — too short to be measured by ordinary methods, and that we 

 can not conveniently express such infinitesimal lengths in fractions 

 of inches or even of millimeters. And as the waves of ultra-violet 

 rays are shorter still, the necessit}' for a special unit of length to be 

 employed in the measurement of such minute waves is obvious. 



Tiie unit most generally used is called the Angstrom unit, and was 

 so named in honor of a Swedish physicist by the name of Angstrom. 



