THE ELECTRICAL RESISTANCE OF THIN LIQUID FILMS. 
453 
the thicknesses corresponding to the colours were not identical with those furnished 
bj Newton’s scale, even when readjusted on account of the omissions already referred 
to. Nor was this surprising. The above description of the method of estimation 
is sufficient to show that in all probability no two observers would agree as to 
the positions of the boundaries between neighbouring tints. If accuracy is required 
each must construct his own scale. 
This might be undertaken by observations on a set of Newton’s rings. Two 
difficulties, however, here arise, the first due to the fact that, even when Newton’s 
rings are produced on a large scale, the colours are far more crowded than on the 
films, and thus not only is their appearance somewhat changed, but differences of 
hue clearly distinguishable in the one case are lost in the other. The second 
difficulty has its origin in the distortion of the lenses, used in the production of 
Newton’s rings, in the neighbourhood of their point of contact. Calculations based 
on the assumed sphericity of the lenses are thus of little use near the central 
black patch. For these reasons it was determined to supplement the observations 
on Newton’s rings by those made on the films, to determine the thicknesses 
corresponding to a few clearly marked colours by the first, and the ratios of the 
thicknesses of the intermediate tints by the second. 
As it was necessary to produce the rings on a large scale, a pair of the curved 
plates, sold for this purpose by Mr. Ladd in a brass case, were placed in a vertical 
plane. The diameters were measured by the cathetometer. The light employed was 
reflected from a mirror and passed through a, slit about two feet distant from the 
rings. By placing a soda flame behind the slit, daylight and homogeneous light 
incident at the same angle could be used in turn. If T be the thickness corre¬ 
sponding to a ring of colour of diameter A, which lies between the ?i th and (n-\- l) th 
dark rings formed by the soda flame, the diameters of which are S M and S« +1 , 
By thus deducing the value of T from the diameters of the dark “soda” rings in 
its immediate neighbourhood, errors due to the non-sphericity of the lens are much 
diminished, and the observation is independent of the closeness of contact between 
the two plates of glass in the centre of the system. 
To obtain accurate results in the neighbourhood of the first dark soda ring it 
was necessary to determine some additional fixed points. A plate of blue cobalt 
glass was therefore interposed between the mirror and the rings. The light thus 
obtained was not homogeneous, but was sufficiently nearly so to make it safe to 
assume that the thicknesses corresponding to the first few dark rings were propor¬ 
tional to the natural numbers. 
The diameters of the four smallest rings were measured, the wave length of the 
light was computed from the larger pair by means of the above formula, and this 
