THE WORLD BEYOND OUR SENSES 



thousands. Calling in the mechanical aid of the 

 photographic plate, an international star - map is 

 now being made which will definitely locate the 

 position of twenty to thirty million suns. That is 

 the difference between the eye and the camera. In 

 some part, these twenty-odd million stars are fixed 

 by means of the invisible rays of the ultra-violet, to 

 which the eye is wholly insensible. 



Again, it is with difficulty that we realize a change 

 in temperature until that change amounts to sev- 

 eral degrees on the thermometer. In order to de- 

 tect and map the invisible rays of heat, the infra- 

 red rays, it was needful to construct instruments 

 about one million times as sensitive. The most del- 

 icate of these devices is the bolometer, which was 

 the invention of Professor S. P. Langley, secretary 

 of the Smithsonian Institution at Washington. It 

 will accurately register a change in temperature of 

 one-millionth of a degree centigrade. It will reg- 

 ister the heat of a candle a mile and a half distant. 



Lest any one surmise that there is some guess- 

 work about this, it may be noted that there are 

 several heat measurers, of almost equal sensibility, 

 whose results accord perfectly with Professor Lang- 

 ley's extraordinary machine. 



From the longest heat-rays, measuring 70,000 /iji, 

 to the shortest known rays of the ultra-violet, but 

 100 fifi in length, we have an unbroken series of 

 4 49 



