NEW CONCEPTIONS IN SCIENCE 



means has yet been found to measure the wave- 

 length of this new kind of light. 



Beyond the other end of the spectrum the meas- 

 uring hand has gone much farther. The longest 

 light- wave is but 810 micro-microns; the longest 

 heat-wave is 70,000. Here are the materials for a 

 scale. This will give you an idea of the compass of 

 the rays upon which we depend for the most of our 

 knowledge of things outside us : 



The visible part of the spectrum is shown by the unshaded space, a all 

 the rest is invisible, and known to us only by mechanical means. 



It is only through the little gap marked a that 

 we get a glimpse of the real world. In order that 

 we might make use of all this wide range of ether- 

 waves, it would be needful that we have a tem- 

 perature sense as acute as our sense of sight, and 

 eyes as sensitive as a photographer's plate. We 

 have neither. A comparison may help to make 

 this clearer. The naked eye, in clear skies, might 

 count in all the heavens perhaps two to four thou- 

 sand stars; the number would vary greatly with 

 individuals and climes. Aided by the finest tele- 

 scopes, this number rises to tens of hundreds of 



48 



