648 
PROFESSORS A. W. REINOLD AND A. W. RUCKER 
coulcl be made with considerable precision. The difficulty was greater when it became 
black, but even in this case the accuracy attainable was greater than might have been 
expected considering the small amount of light reflected. 
The liquid used was, unless the contrary is stated, made with Potash Soap (Brit. 
Pharm.). Its composition has been described by us in the paper on the influence of 
the electric current on the rate of thinning of films previously referred to. Its 
refractive index was P3366 for the sodium line D. 
As nothing in this paper depends on the very accurate measurement of the 
thickness of the films, we estimated it approximately by noting the colour near the 
edge, so that the light was nearly at grazing incidence. The thickness corresponding 
to any colour may therefore be taken as one and a half times greater than if the 
incidence had been normal. The error made in this assumption is probably not more 
than 2 per cent. Inasmuch also as we had to deal with much thinner films than those 
used in our experiments on the electrical conductivity of the films, the difficulty of 
estimating the colour accurately was much greater. 
The thickness of the film indicated by a certain colour is thus l‘5/l‘3366 = 9/8, very 
nearly, of that of a plate of air showing the same colour by means of light falling upon 
it at normal incidence. 
In the following Table, Column I. contains the name of the colour, Column II. the 
symbol of the middle of the colour on the system previously adopted by us. In these 
symbols the letter is the first letter of the name of the colour, the first figure in the 
brackets indicates the order. The colour is divided into ten parts, numbered 0, 1, 2, &c., 
in the order of increasing thickness, and the second number indicates the part of the 
colour displayed. In the third column are the thicknesses expressed in millionths of 
a millimetre, corresponding to the colours when displayed in a thin plate of air at 
normal incidence. In the first order these numbers are (with the exception of the red 
and black) taken from Newton’s table. They are probably only very roughly accurate. 
The other orders are taken from our own table/"' They are only approximate below 
the blue of the second order. Above that colour they are probably correct to 1 per 
cent. In the fourth column are the same numbers as in Column III., each increased 
by one-eighth. They give the thickness of the film corresponding to a given colour 
when mentioned in the present paper. 
* ‘Phil. Trans.,’ vol. 172 (Part 2, 1881), p. 456. 
