Investigation of Superficial Resonance of Sodium Vapour. 1029 



simple photometer which consisted o£ a thin scale of silvered 

 glass E, with a razor edge, made by silvering a piece of 

 plate glass, polishing it, and then striking the edge with a 

 hammer. This mirror reflected the comparison source A to 

 the eye through a cell H containing a solution of bichromate 

 of potash (to remove the green and blue rays of the Bunsen 

 flame). Behind the sharp edge of the silver mirror the 

 illuminated surfaces of magnesia and sodium vapour could be 

 seen at the same time, and by adjusting the Nicol prism C 

 the edge of the mirror could be made to disappear, first when 

 seen against the magnesia and secondly against the back- 

 ground of resonating vapour. The intensities of the two 

 surfaces are then in the ratio of the squares of the angles 

 through which C is turned from the position of extinction. 

 The temperature to which the bulb was heated by the ascend- 

 ing current of hot air was about 330°, measured with a 

 nitrogen mercury thermometer. 



The results are given in the following table, the concentra- 

 tions of the salt solution in the atomizer bulb in the first 

 column, the angles of the Nicol prism C in the next two 

 columns, and the ratio in the fourth. 



Solution 

 Concentration. 



; Angle a of Nicol 

 ; for Resonance 

 Radiation. 



Angle a of Nicol 

 for Magnesia. 



r, ,. sin 2 n' 

 Silica 

 V (Magnesia) 

 I Sodium vapour. 



1 



2048 



3° 

 9°-25 



6° 



4 

 4-5 



1 

 1024 



19° 



1 

 512 



10°-5 



22°-5 



4-8 



1 



256 



13°-5 



36°-4 



0-3 



1 



128 



1 

 64 



14°-4 



45° 

 67° 



96 



15 



14° 



1 

 32 



12°6 



90° 



19 



We see from this table that even with the most dilute 

 solution the diffuse reflecting power of the magnesia is four 

 Phil. Mag. S. 6. Vol. 27. No. 162. June 1914. 3 Y 



