242 
I)R. T. R. MERTON AND PROF. J. W. NICHOLSON ON 
appropriate angle to reduce the intensity of the light to one-tenth of its value in the 
previous exposure. 
It is evident that the differences in the lengths of corresponding lines in the two 
exposures correspond to a reduction of intensity to one-tenth of its value, and are, 
therefore, the differences which would be produced by a step of density equal to l’O 
on the wedge for each of the various radiations measured. The calculation of the 
actual density of the wedge for each radiation then follows obvious lines. The 
following results were obtained for the wedge used in these experiments. Under A 
are given the wave-lengths of the Helium lines which it was found convenient to use, 
and under D A the corresponding densities at the thick end of the wedge. 
A. 
D x. 
A. 
D 
6678 
4-1 
4471 
5-9 
5876 
4-5 
3888 
8-0 
5015 
5-0 
— 
— 
These values were plotted on squared paper, and from the curve thus obtained 
(fig. l) which is quite regular, the density of the wedge for any intermediate wave¬ 
length can be found. There is no very convenient interpolation formula over the 
» Wave-length 
Fig. 1. 
entire region, although the first four values of D x are nearly in the inverse ratio of 
the wave-lengths. But the graphical method is sufficiently accurate. 
(V.) Determination of the Intensities of Lines in Terms of the Intensity of the 
Radiation from the Positive Crater of the Carbon Arc. 
The reduction of photographic to absolute intensities implies the existence of a 
photographic record of some standard source of radiation extending over the whole 
region of wave-length examined. As a standard of radiation we have adopted the 
positive crater of the carbon arc burning in air at atmospheric pressure, and have 
