112 Mr. H. Jeffreys on Periodic 



Examination of these results shows that the predictions of 

 the theory are borne out quite as well as might be expected 

 from the nature of the data used for the calculations, re- 

 membering that the colour sensation and hue data vary from 

 individual to individual in quite marked manner, so that exact 

 calculations could never be made in the manner indicated 

 without a complete study of all the colour characteristics of 

 the eye of the particular observer used, for the field size and 

 level of brightness chosen. The main point at issue, whether 

 the critical speed curve is a luminosity or hue difference 

 phenomenon, is very clearly answered by the results obtained 

 from the red and blue comparison light sources, with their 

 speed minima in the red and blue of the spectrum, as was to 

 be expected. The higher speeds for the blue end as against 

 the red end of the spectrum in using the yellow-white 

 comparison lamp are also exactly in line with the theory 

 here given. 



Summary, 



The subject-matter of this paper may be briefly summarized 

 in the statement that a method of calculating critical speeds 

 of disappearance of colour flicker from colour sensation and 

 hue discrimination data has been developed and tested by 

 experiment. 



I take pleasure in acknowledging the assistance of 

 Mr. E. R. Morton in the construction of the apparatus used, 

 and of Mr. E. F. Kingsbury and Dr. E. Karrer in securing 

 the readings. 



The United Gas Improvement Co., 



Phvsical Laboratory, 



Nov. 22nd, 1916. 



X. On Periodic Convection Currents in the Atmosphere. 

 By Harold Jeffreys, M.A., Af.Sc, Fellow of St. Johns 

 College, Cambridge *. 



IN the mathematical treatment of the motion of the 

 atmosphere, it is customary for meteorologists to 

 neglect in the equation of vertical motion the terms de- 

 pending on the velocity. This is implied in the hypothesis 

 that the barometric pressure at any point is the weight of a 

 vertical column of air of unit cross-section and infinite 

 height, with its lower extremity at the point considered. 

 * Communicated by the Author. 



