58 l\fr. James Rice on 



each producing on the radiometer (if set in the correct position to 

 receive it) an effect denoting the intensity in which it existed in 

 the original heterogeneous stream. We can say that these quasi- 

 heterogeneous streams existed in the original one, just in the 

 same sense that we say that the violet, blue, green, yellow and 

 red of the spectrum existed in the original beam of sunlight before 

 it was analysed by prism or grating. By these means physicists 

 have actually been able to isolate from radiant energy emitted by 

 various light sources, radiations extending as far into the ultra- 

 violet as -06 micron, and as far into the infra-red as 300 microns, 

 or -3 millimetre, which correspond to limits of frequency of about 

 5,000 billions per sec. and 1 billion per sec respectively. It will 

 be, of course, readily grasped that the dimensions of the gratings 

 used for such extremes must be varied with the qualities 

 measured ; for the spacing of the grating must always bear a 

 close relation to the wave-length measured. 



This range of wave-length by no means completes the 

 picture — far from it. The absoi'bing powers which the materials 

 of our apparatus possess for all qualities of radiation, and for 

 some qualities in a very high degree, prevent us at present 

 extending research in the immediate neighbourhood of the limits 

 mentioned. But that radiations of longer and shorter wave 

 length exist there is absolutely no doubt. The ethereal waves 

 of wireless telegraphy are extreme examples of long wave-length. 

 The huge antennae employed to-day emit radiations whose wave- 

 lengths attain in some cases to 3 kilometres, corresponding to a 

 frequency as low as 100,000 per sec. When Hertz first isolated 

 electrical waves in 1887 and verified Maxwells prediction, he 

 worked with waves about 60 cms. long, small enough compared with 

 those that flash our messages across the oceans, but enormously 

 long compared even with the longest of the so-called infra-red 

 waves detected from ordinary sources of light. But just as 

 some workers, by increasing the size of the emitting api)aratus, 

 have produced electrical waves of great wave-length for purposes 

 of communication, others by diminishing the size have sought to 



