POTASSIUM. RUBIDIUM, CAESIUM, AND LITHIUM 



565 



spectrum in the form of Fraunhofer lines may be obtained, as will be 

 explained directly In order to clearly understand the phenomenon of 



the ordinary laboratory spectroscopes 

 with one prism, even with all possible 

 precision of arrangement and with a 

 brilliancy of light permitting the 

 observations being made with a very 

 narrow aperture, the lines whose wave- 

 lengths only differ by 2-3 millionths 

 of a millimetre, are blurred together ; 

 and with a wide aperture a series of 

 lines differing by even as much as 

 20 millionths of a millimetre appear 

 as one wide line. With a faint light 

 (that is, with a small quantity of 

 light entering into the spectroscope) 

 only the most brilliant lines are 

 clearly visible. The length of the 

 lines does not always correspond 

 with their brilliancy. According to 

 Lockyer this length is determined 

 by placing the carbon electrodes 

 (between which the incandescent 

 vapours of the metala are formed), 

 not horizontally to the slit (as they 

 are generally placed, to give more 

 light), but vertically to it. Then 

 certain lines appear long and others 

 short. As a rule (Lockyer, Dewar, 

 Cornu), the longest lines are those 

 with which it is easiest to obtain 

 reversed spectra (see later). Conse- 

 quently, these lines are the most 

 characteristic. Only the longest and 

 most brilliant are given in our table, 

 which is composed on the basis of 

 a collection of the data at our dis- 

 posal for bright spectra of the in' 

 candescent and rarefied vapours of 

 the elements. As the spectra change 

 with great variations of temperature 

 and vapour density (the faint lines 

 become brilliant whilst the bright 

 lines sometimes disappear), which is 

 particularly clear from Ciamician'a 

 researches on the halogens, until 

 the method of observation and the 

 theory of the subject are enlarged, 

 particular theoretical importance 

 should not be given to the wave-lengths 

 showing the maximum brilliancy, 

 which only possess a practical 

 significance in the common methods 

 of spectroscopic observations. In 



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