1030 M. L. Dunoyer and Prof. Wood on Photometric 



times as great as that of the resonating sodium vapour, for 

 the total yellow light o£ the flame. This is of course due to 

 the circumstance that the magnesia reflects all of the D light,, 

 while the vapour scatters only the light corresponding to 

 the cores of lines, the light of the edges of the lines being- 

 transmitted. As the concentration increases the intensity of 

 the resonance radiation increases but slightly after a certain 

 point is reached, since the gain in the intensity of the sodium 

 flame then results chiefly from a widening of the lines. 



For the most concentrated solution (—) the magnesia was 



19 times as intense as the vapour. On reducing the air- 

 current until the yellow colour of the flame was barely visible, 

 a ratio of 3 was obtained, the values of the angles being 

 2° and 3°'5. This result was, however, open to question on. 

 account of the faintness of the light. 



The above results are in accord with those previously 

 obtained by one of us by a different method *. 



If now the molecular resonators absorb none of the light 

 which they abstract from the exciting beam, we ought, if the 

 exciting radiations are made sufficiently homogeneous, to 

 have all of the light diffusely reflected by the vapour ; in 

 other words, our ratio ought to sink to unity when the D 

 lines in the source become infinitely narrow. It is impossible 

 to reach this point by diminishing the amount of sodium 

 in the flame, our lowest value for the ratio being four, or 

 perhaps three. 



We have, however, investigated the matter by employing 

 the principle of the resonance lamp previously described by 

 one of us, which has been used in the investigations on the 

 resonance of mercury vapour. The experiment was made 

 by utilizing the spot of superficial resonance as a source of 

 light for exciting the vapour at a different point on the 

 surface of the bulb. The arrangement of the apparatus is 

 shown in fig. 3. A small triangular spot of magnesia was 

 formed on the surface of the sodium bulb with a black dot 

 of lamp-black at its centre to indicate its position. The 

 image of the sodium flame was thrown upon this spot, and 

 the magnesia triangle shone brilliantly upon the less intense 

 background of the resonance radiation (tig. 3, a). The bulb 

 was now rotated until the triangle of magnesia was in dark- 

 ness, and an image of the spot of resonance radiation thrown 

 upon it by means of a large concave mirror, formed by 



* L. Dunoyer, Journal de Physique, iv. p. 17 (1914). 



