February i6, i 



NA TURE 



573 



The usual laboratory arrangement for observing the inch. The unit of wave-length usually employed is the 

 spectra of flames is shown m the woodcuts. | ten-millionth of a millimetre. These wave-lengths get 



Further, the system of images of the needle (or slit) | shorter as we pass from the red to the violet. 



So much then in general for the radia- 

 tions given out by light sources, and the 

 manner in which the spectroscope shows 

 them, and the student records their 

 positions. 



Spectrum analysis was established 

 when experiment proved that no two 

 substances which give a line spectrum 

 give the same order of lines from one 

 end of the spectrum to the other; in other 

 words, the line spectrum of each chem- 

 ical substance differs from that given by 

 anv other 



Fig. 4.— The spectrum of .1 complicited light-source .is seen with a circular and a line slit. t_r .1 • r 1 ... 



Here then is one of the secrets of the 



new power of investigation of which the 



varies for each substance, and it is on this ground that spectroscope has put us in possession : we can recognise 



the term spectrum analysis is used, because we can in each element by" its spectrum, whether that spectrum is 



this way recognise the various substances in the flame. 



But we are not limited to flame temperatures ; sub- 

 stances in a state of gas or vapour may be made to glow 

 by electricity. At these higher temperatures very com- 

 plicated spectra are produced, and again the spectrum 

 is special to each chemical substance experimented on; 

 the images of the needle (or slit), occupying different 

 positions along the spectrum according to the nature of 

 the source of light. 



F'?- 3 gives us a laboratory prism spectroscope of 

 small dispersion ; with the more complicated spectra the 

 phenomena are often better seen if more than one prism 

 are employed. Fig. 5 shows an instrument in which four 

 prisms are used. 



For accurate measures of the wave-lengths of the lines 

 a grating is employed as shown in Fig 6. 



It is in the case of the more complicated spectra that 

 the wave-length has to be specially considered from the 

 point of view of defining the position of a line. It is not 

 enough to say, as was said in the case of the sodium line, 

 that it is located in the orange. 



The lengths of the various light-waves are very small. 1 i-.^ « a . - ■ 



T^i 1 .u /- 1 J r .L -jji /- riG. 6. — Angstrom s grating spectrometer. 



Ine wave-length of the sound-wave of the middle C 

 of a piano is about 4 feet, while the wave-length of yellow produced in the laboratory or is given by light travelling 

 light as defined by that of a line very accurately measured ' earthwards from the most distant star, pro't'ided the 



element exists both here and there. 



It is in this way that spectrum analysis 

 helps us with regard to chemistry ; the 

 spectrum varies according to the chemical 

 substance which produces it. 



Flutings. 

 The earliest spectroscopic observations 

 revealed the fact that in some spectra the 

 lines, instead of being irregularly distri- 

 buted along the spectrum, were arianged 

 in an easily seen rhythmic fashion. Such 

 allocations of lines are called flutings, as a 

 succession of them gives rise to an ap- 

 pearance strongly recalling the flutings of 

 a Corinthian column seen under a strong 

 side light. 



Our improvised spectroscope helps us 

 here too ; use the candle and straight slit 

 in front of it as before, but shorten the 

 slit, and only allow the blue light from the 

 base of the candle flame to pass through 

 it to the prism. We see two or three 

 sets of flutings. These are the flutings 

 of carbon, and they are amongst the 

 most beautiful examples known and are 



IS ■0005895 of a millimetre, that is 5895 ten-millionths of I thoroughly typical. Norm.\n Lockver. 



a millimetre; so that there are 43,130 waves in a British I (^To be continued.) 



NO. 1529, VOL. 59] 



Fig. 5 — Steinhcil spectroscope with foui 



