THE BECQUEREL RAYS 131 



light are comprised in less than an octave; that is, the 

 longest visible waves are not twice as long as the shortest 

 visible. Their length, moreover, is not inconceivably 

 minute. The twenty-fifth part of an inch, or a millimetre, 

 although a small distance, is easily seen with the naked 

 eye ; indeed, the twentieth part of that length can still be 

 estimated without the aid of a lens. The average length 

 of a light- wave is about the hundredth part of that dis- 

 tance, or about the two-thousandth of a millimetre. The 

 thousandth part of a millimetre is termed a micron, the 

 symbol for which is the Greek letter //, ; the wave-length 

 of deep red light is f/z, and of violet light f//,. 



There are, however, ethereal waves which cannot be 

 seen. Those of greater wave-length give rise to the sensa- 

 tion of heat ; they are termed ' infra-red ' waves ; while 

 those of shorter period are accessible to photography, for 

 they change the nature of the compounds of silver which 

 form the sensitive coating of a photographic plate, and 

 can thus be recognised ; they are termed ' ultra-violet ' 

 waves. One of the difficulties of tracing the existence of 

 the short wave-lengths by photography consists in the 

 absorptive power which glass and air have for such waves. 

 A pane of glass, though transparent to ordinary light 

 waves, is nearly opaque to ultra-violet waves. Quartz or 

 crystal, of which spectacle-lenses are generally made, is 

 much more transparent to vibrations of short wave- 

 lengths ; but even quartz has its limits. By an ingenious 

 contrivance for exposing a sensitive plate in a vacuum, so 

 that the absorption of the air did not influence the result, 

 Schumann succeeded in chronicling the existence of waves 

 only -^Q/JU in length. On the other hand, Langley, by 

 means of an exceedingly delicate apparatus for detecting 

 heat-vibrations, termed a bolometer, has detected waves 

 as long as 30 p. Between that wave-length and one two 

 hundred times as great, six millimetres, there is a gap in 



