DISTRIBUTION OF INTENSITY IN BROADENED SPECTRUM LINES. 
463 
different density is obtained by doubling the intensity of the light and halving the 
time of exposure. 
(III.) The sensibility of the photographic plate varies for different wave-lengths. 
This, of course, depends on the particular kind of plate used, but may be neglected 
in the investigation of broadened emission lines, since the sensibility of the plate may 
be taken as constant over the short range of wave-length covered. 
(IY.) Irradiation, or spreading of the image on the photographic plate, which has 
recently been the subject of a quantitative investigation by TugmanY Owing to 
the scattering of light by the grain of the plate the size of a photographic image 
increases with the time of exposure or the intensity of the light. 
It is therefore evident that if quantitative measurements are to be made by a 
photographic method, the method adopted must comply with the principle,f that two 
sources of light or two regions of illumination can only be considered to be of equal 
intensity when they produce the same degree of density in the same time on portions 
of the same photographic plate. It is believed that the method adopted in the 
present investigation fulfils these conditions and is independent of the eccentricities 
of the photographic plate. 
An accurate wedge of neutral-tinted glass, cemented to a similar wedge of clear glass 
so as to form a plane parallel plate, was mounted immediately in front of the slit of 
a spectroscope, in the manner commonly used for determining the sensibility curves of 
photographic plates. Under these conditions the spectrum of a discontinuous source 
thrown on to the slit through the neutral wedge is seen to consist of lines which are 
bright at one end, corresponding to the thin end of the wedge, and gradually fade off 
towards the region corresponding to the thick end of the wedge. The apparent 
length of a line depends on its intensity, and the relative intensity of two adjacent 
lines can be determined by measuring the lengths at which they can just be seen. 
Since in broadened spectrum lines the intensity, generally speaking, falls off more 
or less regularly from the maximum of intensity, a broadened line appears, with 
the arrangement described, as a wedge, the apex of which corresponds to the point 
of maximum intensity. By a measurement of the shape of the photographed image 
of such a wedge, it is possible to calculate the distribution of intensity in the 
broadened line. It is only necessary to pick out points of any convenient density 
which can be recognised, and to measure their height from the base of the wedge. 
The method therefore conforms with the conditions that have been laid down, and is 
independent of the eccentricities of the particular plate used or its subsequent 
treatment. Since the density which can be most easily recognised is small, it is 
evident that the results will not be vitiated by irradiation provided that the spreading 
due to irradiation from the centre of the base of the wedge is small in comparison 
* ‘ Astrophys. Journ.,’ 42, p. 331, 1915. 
f Cf. Houstoun, ‘Roy, Soe. Edinburgh Proc.,’ 31, p. 521; 1911. 
