1883.] 



On the Total Solar Eclipse of May 17, 1882. 



151 



The electrical observations afford a means of comparing the 

 thicknesses of different black films, and observing whether the 

 thickness of the black portion of any particular film alters as its area 

 increases. The results obtained in the paper, and in a previous pre- 

 liminary investigation on the same subject (" Proc. Roy. Soc," 1877, 

 No. 182, p. 334), are summed up by the authors as follows : — 



(1.) Persistent soap films, which thin sufficiently to exhibit the 

 black of the first order of Newton's rings, invariably display an 

 apparent discontinuity in their thickness at the boundary of the black 

 and coloured portions. 



(2.) The whole of the black region at the time of, or very soon after, 

 its formation, is of uniform thickness. 



(3.) This thickness remains unaltered in any film, whether the 

 coloured parts of the film are thinning or thickening, increasing or 

 diminishing in extent. 



(4.) It is different for different films, but no connexion has been 

 traced between its magnitude and the time which elapses between the 

 first formation of the film and the first appearance of the black, or 

 between either of these and the time of observation. 



(5.) The mean values of this thickness are the same to within a 

 fraction of a millionth of a millimetre, whether the films are plane or 

 cylindrical, in contact with metal or with glass, formed of soap 

 solution alone, or with the addition of more than two-thirds of its 

 volume of glycerine. 



(6.) Two totally independent methods of measuring the thickness of 

 the black portions of the films give concordant results. 



(7.) The mean value of the thickness calculated by giving equal 

 weight to the results of the electrical and optical experiments is 

 11*6 X 10 -6 mm. The extreme values were 7*2 x 10~ 6 and 

 14-5 X 10- 6 mm. 



The smaller of these quantities is therefore a limiting thickness to 

 which a soap film in air saturated with the vapour of the liquid from 

 which it is formed rarely attains, and below which none of the films 

 observed by us have thinned. 



III. " On the Total Solar Eclipse of May 17, 1882." By Arthur 

 Schuster, Ph.D., F.R.S., and Captain W. de W. Abney, 

 R.E., F.R.S. Received April 9, 1883. 



(Abstract.) 



The first part of this paper gives an account of the journey and 

 preparations for the eclipse. Three instruments were to be used 

 during totality. 



