462 
PROF. J. W. NICHOLSON ANI) DR. T. R. MERTON ON THE 
obtained for the hydrogen lines H a , H^, and H y were qualitatively in accordance with 
the intensity distribution to be expected on the view that the electric field of 
neighbouring atoms was responsible for the broadening. The present investigation is 
concerned with the quantitative determination of the distribution of intensity in 
broadened spectrum lines, with the view of throwing some further light on the nature 
of the broadening and the circumstances which control it. 
(III.) The Methods of Measurement. 
It is at once evident from the complex nature of the phenomena that a measurement 
of the limiting' order of interference at which fringes can be seen for the broadened 
lines will bt# of little value in determining the intensity distribution. If the intensity 
distribution could be exactly predicted on theoretical grounds, it might be possible to 
verify the theory by measurements with the interferometer, but even in this case 
serious difficulties might arise if the intensity distribution curve were not of some 
simple form. In a recent investigation King and Koch* have described a method 
of investigating the structure of spectrum lines. The method adopted by these 
investigators consists in photographing the spectrum under a high dispersion, and in 
obtaining a curve relating the density of the image on the photographic plate to the 
wave-length. This is accomplished by causing the plate to move slowly in front of a 
slit through which light from a constant source is passed, and by continuously recording 
the resulting changes in the intensity of this light by a method involving the use of 
the photo-electric cell. The method has yielded valuable results in the study of the 
variation in the character of spectrum lines under different conditions of excitation, 
but it would appear difficult to employ a method of this kind to determine the 
quantitative intensity distribution in the lines on account of the eccentric and 
somewhat anomalous relations which determine the form and density of photographic 
images. In adopting a photographic method for quantitative investigation the 
following phenomena have to be taken into account 
(I.) There is no linear relation, and indeed no very definite relation, between the 
density of the image on a photographic plate and either the intensity of the light 
which produces the image or the duration of the exposure. The relation varies with 
different brands of plate and is affected by the chemical treatment of the plate in 
developing the image. An extreme case occurs when the image becomes solarized, 
or over-exposed to such a degree that it is no longer capable of development, a 
phenomenon well known to spectroscopists as the cause of the spurious reversal of 
spectrum lines. 
(II.) The product of the intensity of the light by the time of exposure does not 
produce a constant value of the density of the photographic image, that is to say, a 
* ‘Astrophys. Journ.,’ 39, p. 213, 1914. 
