Spectrum Lines for Monochromatic Illumination. 525 



this way. Even in the case of the three green copper lines, 

 I found the greatest difficulty in getting sufficient illumina- 

 tion with a single line isolated by means of a very large 

 monochromator of 1*5 metres focus. 



In the present paper I shall give a method which enables 

 us to utilize a source of light of large size, say 1x3 cm., 

 and remove one or more lines from it with practically no loss 

 of light. 



For example, we can form two images of a sodium flame by 

 means of a condenser having an effective aperture equal to/2, 

 one image containing only the light of wave-length 5890, 

 the other only light of wave-length 5896, both images being- 

 very nearly as intense (with respect to one sodium line) as 

 if the condenser had been employed without the separating 

 apparatus. 



The method is an improvement upon one which I used 

 many years ago in the study of the dispersion of sodium 

 vapour and described briefly at that time. It is a polarization 

 method, and may be described briefly as follows. 



If plane-polarized monochromatic light is passed through 

 a plate of some doubly refracting crystal with its direction 

 of vibration making an angle of 45° with the axis, it will 

 emerge plane-polarized parallel to the original plane for 

 certain thicknesses of the plate, and plane-polarized at a 

 right angle to this plane for other thicknesses. For inter- 

 mediate thicknesses it will be elliptically or circularly 

 polarized. 



If we employ a plate of quartz 30 mm. thick the emergent 

 waves of Di and D 2 of sodium will be plane-polarized at 

 right angles to each other, and either can be quenched by a 

 nicol suitably oriented. If white light is used, and analysed 

 by a spectroscope, the spectrum will be furrowed by dark 

 bands, the distance between a bright and a dark band being, 

 in the yellow region, -6 Angstrom units, the distance between 

 the D lines. 



As it was desired to utilize this principle for the separation 

 of the D lines for the purpose of exciting the resonance 

 radiation of sodium vapour by the light of D t and D 2 

 separately, by which means we may determine whether the 

 mechanisms which give rise to the radiations are coupled 

 together, an investigation which is being carried to a suc- 

 cessful conclusion in collaboration with L. Dunoyer at the 

 present time, it became necessary to bring the method up to 

 the highest possible efficiency. As it is necessary to employ 

 a large condenser and work with very divergent and con- 

 vergent cones of light, a block of quartz of very large size 



Phil. Maq. S. 6. Vol. 27. No. 159. March 1911.. 2 N 



