Four Transition Points in the Spectrum. 55 



If vapour is given off from the drop it will condense to form 

 a primary composite surface, and this being contractile may 

 pnli the drop itself out to form a secondary surface. The 

 fact that B.P. " paraffin " and castor oil do not spread at all 

 on clean glass is due to their low vapour pressure, and is no 

 evidence as to the tension of- the glass- oil interface. 



I confess I should not have expected that a film of fluid of 

 the order of 1 micron in thickness on a solid face would be 

 spontaneously contractile, but the migration of the drops of 

 acetic acid appears to afford conclusive evidence on this 

 point. Any primary composite surface will contract only if 

 a further thickening of the film lowers the surface tension. 

 A fully formed primary surface, that is to say one in which 



the film has thickened to the point at which the function -7— 



ax 



(where T is the tension, and x the thickness of the film) 

 changes its sign, would have no tendency to contract; indeed 

 it would resist further contraction. The fact that primary 

 surfaces saturated with fluid actually do contract sponta- 

 neously when exposed to air, is due frequently to the thinning 

 of the surface film by evaporation. This is readily demon- 

 strable with pure benzene on water. Substances such as 

 benzene on water and acetic acid on glass behave in fact as 

 though the film on a primary composite surface varied in 

 thickness with variations in the vapour pressure of the 

 fluid of which it is composed. If this be so the film on a 

 composite surface cannot be always and of necessity one 

 molecule thick as some writers suppose. 



IV, On, the Discovery of Four Transition Points in the 

 Spectrum and the Primary Colour Sensations. By Frank 

 Axlen, Ph.D., F.R.S.C., Professor of Physics, University 

 of Manitoba, TI innipeg * . 



ri 1HE researches in colour-vision which I have made from 

 A. time to time have had for their secondary object the 

 extension of our knowledge of the persistence of vision, and for 

 their primary object the determination in an incontrovertible 

 manner, if possible, of the number of fundamental colour- 

 sensations, and the precise waves of light by which they are 

 excited, concerning which there has been and still is much 

 controversy. 



In this communication I have gathered together some 



* Communicated by the Author. 



