44 D R W. GEOFFREY DUFFIELD ON THE 



A photograph of this part of the spectrum has been taken at 200 atmospheres, and 

 here the spectrum looks still more continuous. 



The breadths of the bands at different pressures are given in Tab] II., p. 4 

 measurements are in Angstrom units. 



(3) Table of Wave-Lengths and Intensities of Bands. -This is given on pp. 45-47 



(4) The Relative Intensities of the Bands under Pram*. Though the intensities 

 of the bright bands are increased by pressure, it does not produce any marked change 

 in the relative intensities of adjacent bands. What is, however, noticeable is the 

 gradual increase in the intensities of the bands that are distant from the strong lines 

 a and c relatively to those in their immediate neighbourhood, e.g., band at 4162 has 

 increased in intensity relatively to 4199 and 4215, which are near the line c at 4212. 

 For other examples see Table II. There is an outward extension of the luminosity 

 from these lines as the pressure increases. A line not shown on the photographs in 

 the red (ride p. 50) seems also to be concerned in the production of the spectrum. 



11. The Continuous Spectrum. The highest pressure available for these experi- 

 ments was 200 atmospheres, and one photograph was obtained at this pressure, 

 but no bands are discernible against the background of continuous spectrum. Too 

 much risk would be run by pushing these experiments further with the existing 

 apparatus, but it would be of interest to observe if this continuous spectrum persists 

 at still higher pressures and behaves like the continuous spectrum derived from black 

 body radiation, i.e., if the maximum of intensity is displaced towards the region of 

 short wave-lengths as the temperature increases, or if the energy remains localised in 

 the centres of a hidden banded spectrum from which the continuous spectrum has 

 been developed. 



12. Influence of the Electrical Conditions upon the Banded and Continuous 

 Spectra. It has already been remarked that at the moment of extinction the bright 

 lines have been seen to flash out and replace the bands. The electrical conditions 

 accompanying the sudden breaking of the circuit occasion this change. Visual 

 observations suggest that at such times the quantity of material taking part in the 

 discharge is diminished ; for example, when the arc is burning steadily under 

 pressure, one of the yellowish-green lines is broad, almost resembling a band, and is 

 strongly reversed, yet at the moment when the arc is extinguished this line narrows 

 down to the width of the reversal and flashes out as a bright line. By varying the 

 voltage of the discharge, interesting information regarding the electrical conditions 

 necessary for the production of the banded spectrum should be forthcoming. 



The Origin of the Banded and Continuous Spectra. The resemblance of the 



banded to an absorption spectrum requires discussion ; hitherto it has been treated as 



a emission spectrum. It is necessary to inquire into the possibility of the bands 



mg the result of a continuous radiation from the hot poles of the silver and suffering 



absorption by the surrounding vapours. 



Careful examination has shown that no appreciable continuous spectrum is obtained 



