no 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
the data being taken from the curves drawn by Nutting* and compared 
with the more recent determinations by Ives, the agreement between the 
two observers being very close. On this curve the maximum intensity at 
545 nn is assumed to be unity and the other intensities proportionately 
smaller. A graphical determination of the partial areas included by this 
white light curve indicates that at about 555 nn the total amount of light on 
either side is about equal; in other words, 555 MM is about the center of the 
visible spectrum when measured in terms of total light. 
From this curve of white light visual sensibility, which indicates the 
approximate distribution of light throughout the visible spectrum of 
standard white light, it is a simple matter to calculate the relative inten- 
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Path difference m jiji 
FIG. 70. In this figure the change in intensity of different colors (A = 640, 620.3, 555, 
500, 481. 2 MM) with change in path-difference from o to 1000 MM is illustrated. The 
relative brightness of the different colors is the same as that observed in white light. 
The intensity minimum at about 555 MM is also clearly evident in the figure. 
sities of the different spectrum colors for any given path-difference between 
interfering waves. In plotting the values obtained from the standard 
intensity equation 
/' = I sin 2 
A 
the intensity may be chosen as ordinate and either the path-difference A 
or the wave-length as abscissa; in Fig. 70 the path-difference has been 
chosen for the abscissa and curves for different colors (X = 48i.2, 500, 555, 
620.3, 640/1/4) are plotted, while in Fig. 69 the wave-lengths are the ab- 
scissas and curves for the different path-differences are shown. In Figs. 
69 and 70 the increase in intensity of total light as the path-difference in- 
creases from o to about 300 /4/4 is clearly shown (the interference color pass- 
ing from dark gray to white and pale yellow of the first order) ; from about 
Circular. Bureau of Standards, No. 38, 1911. 
