1883.] On the Limiting Thickness of Liquid Films. 149 



were used, and such a precaution is quite necessary. The photographs 

 were not varnished. A certain number of lines measured by previous 

 observers have been compared with the new measurements. Taking 

 the numbers given by Thalen, Lecoq de Boisbaudran, and Cornu for 

 150 lines in the spectra of magnesium, zinc, cadmium, aluminium, 

 indium, thallium, iron, &c, a close agreement with their measurements 

 affords satisfactory evidence of the accuracy of these determinations. 

 Besides the wave-length, a very careful description of the appearance 

 of each line is given, together with its linear measurement indicating 

 its position on a series of photographs obtained with the prism 

 spectroscope, which series of photographs is presented with the paper. 

 A distinction is drawn between those lines determined directly with 

 the grating and others too faint to be seen on diffraction photographs, 

 which were measured by the aid of the prism spectroscope and an 

 interpolation curve 9| metres in length. The total number of lines 

 measured and described is 2307, namely: — Magnesium, 42 ; zinc, 151 ; 

 cadmium, 141 ; aluminium, 30 ; indium, 104 ; thallium,. 70 ; copper, 

 164 ; silver, 124 ; mercury, 80 ; carbon, 20 ;. tin, 129 ; lead, 86 ; 

 tellurium, 322 ; arsenic, 112 ; antimony, 211 ; bismuth, 156 ; air, 215 ; 

 and iron, 150. 



A series of eighteen enlarged photographs, 36 inches in length, are 

 presented with the paper, on which each line has its wave-length 

 written over it. 



II. " On the Limiting Thickness of Liquid Films." By A. W. 

 Reinold, M.A., Professor of Physics in the Royal Naval 

 College, Greenwich, and A. W. Rucker, M.A., Professor of 

 Physics in the Yorkshire College, Leeds. Received March 6, 

 1883. 



(Abstract.) 



The previous investigations of the authors have shown that the 

 specific electrical resistance of a soap film thicker than 374 X 10~ G 

 mm. is independent of the thickness, and that the composition of 

 films formed of M. Plateau's " liquide glycerique " may be largely 

 altered by the absorption or evaporation of aqueous vapour which 

 attends even slight changes in the temperature or hygrometric state 

 of thq air (" Phil. Trans.," Part II, 1881, p. 447). 



In the present paper they describe a modified form of the apparatus 

 which they previously employed. The glass case in which the films 

 are produced is surrounded by water, and additional precautions are 

 adopted for maintaining the aqueous vapour within it at the tension 

 proper to the liquid of which the films are formed. These changes 



