August 16, 1895.] 



SCIENCE. 



175 



that kind of light which it can itself radiate. 

 (Kirchhoflf's law. ) In the spectrum of sun- 

 light we iind a Fraunhofer line at just the 

 same point as the dark line occurs when 

 the white light is passed through the soda 

 vapor. The more exact the measurement 

 the more perfect is the coincidence, hence 

 we are compelled to conclude that the light 

 from the white-hot body of the sun has 

 passed through hot soda vapor before it 

 reached our prism ; this must have been in 

 the sun's atmosphere, therefore sodium 

 must exist in the sun. The reasoning is 

 analogous and even more convincing that 

 hydrogen with three lines is there, iron with 

 its many hundred, and so of many other of 

 the elements known to us. Some of the 

 Fraunhofer lines do not correspond to any 

 of our elements, but may be identified at 

 any moment, as, for example, those of the re- 

 cently discovered argon and helium. 



The question was soon raised, whether 

 the energy stopped sharplj^ at the red and 

 violet, or extended into the invisible. So- 

 called fluorescent substances soon found 

 something, ' actinism,' beyond the violet, 

 and the thermometer soon found heat be- 

 yond the red, and thus began the campaign 

 into the unknown invisible regions of the 

 ' ultra-violet ' and the ' infra-red.' 



A word as to the size of these little waves 

 and their rapidity of vibration. They 

 are so small that for their measurement a 

 special unit was adopted. The ' micron ' 

 (short) is yjiVjr of a millimeter, or 7755^0 of 

 an inch, and is usually represented by the 

 Greek letter mu, fi. A ' wave ' whose 

 length, including both hill and valley, is 0, 

 75/-/ (ifBiVijiiir inch), and which vibrates only 

 about four hundred thousand million times 

 per second, produces the sensation of red 

 in our eyes. If the rate is seven hundred 

 million times per second and the length 

 about 0, 43// (^sjViJTSTr inch) the sensation 

 will be violet. Between these limits the 

 sensations are the various reds, oranges, 



yellows, greens and blues. Beyond these 

 extremes the vibrations liave no visible ef- 

 fects upon our eyes. 



Photography in the trained hands of Vic- 

 tor Schumann, of Leipzig, has carried the 

 frontier in the ultra-violet out to a wave 

 length of 0, 12//, or a distance more than 

 equal to the whole visible specrum from red 

 to violet. Meanwhile the ' bolometer,' with 

 the consummate manipulation of S. P. Lang- 

 ley, has forced the limits of the infra-red to 

 10, Qpt, with good assurances of waves two 

 or three times as long as that, giving us an 

 inira-red spectrum at least twenty times as 

 long as the entire visible one from violet to 

 red. 



Metals on being heated increase their 

 resistance to the passage of an electric cur- 

 rent ; in this fact lies all the secret of suc- 

 cess in infra-red si^ectrometry. Wheate- 

 stone devised a system of electrical connec- 

 tions with battery and galvanometer, which 

 compares resistances just as a lever balance 

 compares weights. (See Fig. 1.) Let us 



Fig. 1. Diagramatic representation of tlie connec- 

 tions of a Wlieatestone bridge ; a, b, c, d, are the four 

 resistances; B the battery and G the galvanometer. 



suppose two very thin strips of metal to 

 be so arranged in a Wlieatestone balance 

 (or 'bridge' as it is called) (See Fig. 2) ; 

 when the adjustment is correct no cur- 

 rent will run through the galvanometer, 

 and the beam of light reflected from the 

 little mirror attached to the ' needle ' re- 



