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PROFESSORS A. W. REINOLD AND A. W. RUCKER ON 
In the other orders Newton’s two lists and our own agree in distinguishing only 
two colours, green and red. 
It was soon found that a more precise method of nomenclature and a convenient 
system of symbols for the tints were needed. Each colour was therefore sub-divided 
into ten equal parts which were indicated by numerals from 0 to 9 in the order of 
increasing thickness. Any tint could be readily expressed by writing down the first 
letter of its name followed by a bracket containing two numbers, the first of which 
showed the order, and the second the part of the colour referred to. Thus B ' 3, 0,] 
B [3, 9] and B [3, 5] indicate the divisions of the blue of the third order nearest to 
the purple, nearest to the green, and halfway between the boundaries of those colours 
respectively. The last of these corresponds to the thickness answering to the same 
colour in Newton’s scale. 
If a single tint is alone presented to the eye, it is often difficult to assign the 
precise value to be attached to it. The films, however, generally displayed several 
colours in the same vertical line, and it was thus possible to reinforce the judgment as 
to the tint at any point by estimating the distances of that point from the boundaries 
of the colour in which it lay. The increment in thickness corresponding to the entire 
range of a colour, varies considerably, and thus the tenth of a tint has also different 
values in different parts of the scale. As a rule it is less than one per cent, of the 
thickness, though in the blue of the second order it rises to 3 per cent. It was thus 
sometimes necessary and possible to estimate to the twentieth of that colour. 
A valuable check on the accuracy of the observations was afforded by the use of the 
two mirrors. The light reflected from the first was incident on the film at about 45°, 
and the angle of incidence of that reflected from the second was made as small as 
possible and varied on different occasions between 2° and 7°. When the thickness of 
the film at any given level was to be measured, the horizontal spider line of a 
cathetometer-telescope was brought to that level, and the colours were noted at the 
points were it crossed the two bright strips. 
The thicknesses corresponding at normal incidence to the colours at these points (Tj 
and T 2 ), and the true thickness of the film (T) are connected by the equations 
T T 
_ -D _ As 
cos cos if’ 
where and i 9 ' are the angles of incidence on the internal surface of the film. Two 
simultaneous but independent measures of the thickness were thus obtained, the mean 
of which was adopted as correct. 
This method presented the additional advantage of affording a means of checking 
the accuracy of the scale of colours employed. The ratio Tj -fT 2 depends only on the 
nature of the liquid and the magnitude of the angles of incidence. On calculating it 
for a number of observations, it was soon found that although tolerably constant for 
any particular colour, it varied considerably for different colours. This shewed that 
