﻿Changes in 'Diffraction Spectra. 35 



When this cell was placed in front of the wire grating it 

 produced little effect on the spectrum before the vapour was 

 introduced, as both the glass plate and mica were sufficiently 

 uniform in optical thickness; but when it was filled with 

 ether vapour the secondaries on the observer's right in fig. 3 

 became much brighter than before, and those on the other 

 side very faint. 



We did not obtain a photograph showing the full effect 

 owing to the difficulty of keeping the density of the vapour 

 constant for a sufficient length of time; but No. 2, PL IV., 

 taken under these conditions, shows the first secondary on 

 the right much brighter than anything on the other side. 



Summary. 



1. Lord Rayleigh's investigation of the effects of cubic 

 aberration has been extended to the first seven secondary 

 maxima on either side of the primary, with special reference 

 to the secondary maxima in diffraction spectra. 



2. It has been found that a cubic aberration amounting to 

 three-quarters of a wave-length at the edges of the aperture 

 of the echelon we have employed is sufficient to explain why 

 photographs taken with it show a series of secondary maxima 

 on one side of a bright line and not on the other. 



3. The degree of aberration present in an echelon spectrum 

 may readily be determined by measuring the distances of 

 the brightened secondaries from the primary to which they 

 belong. 



4. When the aberration in an echelon spectrum is relatively 

 large the series of brightened secondary maxima provide a 

 convenient scale of intensity which may be found useful in 

 comparing the intensities of lines in the spectrum. 



5. Photographs have been taken showing the effect of an 

 aberration cell on the secondary diffraction maxima of a 

 wire grating. 



In conclusion, we wish to give our best thanks to Professor 

 Rutherford and to Professor Schuster for placing the facilities 

 of the Physical Laboratories at our disposal and for the inteiest 

 they have taken in our work. 



D2 



