380 
DR, T. R. MERTON AND MR. S. BARRATT 
were taken on Ilford Rapid Process Panchromatic plates, which were developed with a 
Hydroquinone and Potash developer which gave very great contrast, and by a slight 
cutting ” of the plates after development with a solution of Potassium Ferricyanide 
the contrast became so great that the positions of the edges of the black lines could 
be measured on a photo-measuring micrometer to a few thousandths of a millimetre. 
The half-widths of the lines were deduced from the measured “ apparent widths 55 in 
the single- and double-order positions. The theory of the method, as involving the use 
of the echelon grating, is given in the succeeding section. With other instruments of 
high resolving power it would be necessary to vary the intensity of the incident light 
in successive exposures, e.g., by the interposition of a filter of known absorbing power 
for the radiation in question. 
(8) Theoretical. 
The theory of the method can be seen from the figure in which the upper curve repre¬ 
sents the distribution of intensity by the echelon which is given by the equation 
1„ = sin 2 «/a 2 , where a = rfor/A, a- denoting the step of the grating, A the wave-length 
and Q the angle of diffraction. 
In the same figure are shown the intensity distribution curve of a spectrum line, as 
seen in the single-order, position, with the two curves showing the line in the double¬ 
order position on either side. Although the actual distribution of intensity in the line 
is given by the equation I = e~ kx \ the observed distribution of intensity differs somewhat 
from this as the distribution of intensity given by the echelon is superposed. 
In the two strips below these curves are shown the appearance of the line as seen 
on the plate, with exposures of equal duration, in the double- and single-order positions 
respectively. (The absence of disturbance due to irradiation can be seen by an 
