796 
Also the height of the weaker maxima on both sides is in harmony 
with the theory, when it is taken into consideration that the accuracy 
is not so great at these small intensities. A considerable deviation 
is observed in the neighbourhood of the position where the intensity 
becomes zero. Here the observed intensity is much greater than 
theory led us to expect. !) As the three lines examined have a very 
different intensity and yet present the same deviation from the 
theoretical curve, this deviation cannot be attributed to a systematic 
error in the determinations of the intensity. It is quite possible 
that the approximations in the theory mentioned before give rise, 
at least partially, to the lack of agreement between experiment and 
theory. When we confine ourselves, however, to the central part of 
the curve, the agreement is sufficient. For the derivation of the true 
intensities from the distribution of intensity in a line-spectrum ob- 
served by means of an echelon it will, therefore, be desirable 
that the lines to be compared lie in the central part of the image 
of diffraction. 
Institute for Theoretical Physics. Physical Laboratory. 
Utrecht, Sept. 1920. 
1) It is, however, also possible that the width of the real distribution of intensity 
is greater than that which follows from the distance of the orders. For the position 
where the intensity becomes zero, lies further from the centre than follows from 
the theory. 
