530 Proceedings of Royal Society of Edinburgh. [sess. 
Fig. 7. 
§ 13. The following table shows calculated values for the ordinates 
of the two curves ; also values (essentially negative) for the formula 
of intensity calculated from the negative values of /z algebraically 
admissible from (11). 
The negative values of g have no physical interpretation for 
either curve ; but the consideration of the algebraic prolongations 
of the curves through the zero of ordinates on the left-hand side of 
the dark band illustrates the character of their contacts. The 
physically interpreted part of each curve ends abruptly at this 
zero ; which for each curve corresponds to a maximum value of 
x. The algebraic prolongation of the /jl curve on the negative 
side is equal and similar to the curve shown on the positive side. 
But the algebraic prolongation of the intensity curve through its 
zero, as shown in the table, differs enormously from the curve 
shown on the positive side. To the degree of approximation to 
which we have gone, the portions of the intensity curve on the 
left and right hand sides of the dark band are essentially equal 
and similar. This proves that so far as Sellmeier’s theory 
represents the facts, the penumbras are equal and similar on 
the two sides of a single dark line of the spectrum uninfluenced 
