Dispersion in Relation to the Electron Theory. 243 



In the ultraviolet, when using a spectrum rich in lines, a 

 choice of two methods may be made. (1) A particular line 

 may be selected and a photograph taken for each one of a 

 number of settings of the analyser. Provided the latter 

 are not too far apart (3' or 4') the particular reading which 

 gives equality of illumination in the upper and lower spectra 

 can be ascertained with the same degree of accuracy as in 

 visual observations. (2) The analyser may be set to a given 

 position, and the particular line with equal density in the 

 two spectra may then be found by inspection. The latitude 

 of error in the former method is clearly independent of the 

 rotary dispersion and depends only upon the light-intensity 

 and the decree of half-shade. But in the latter method, 

 although the accuracy is increased by reducing the angle of 

 half-shade, it also depends to a large extent upon the 

 luminosity-gradient along the spectrum, and therefore upon 

 the rotary dispersion. If this is small, as it is with small 

 magnetic potential, there is difficulty in locating the position 

 of exact equality of illumination. For obtaining the rotation 

 corresponding to an accurately known wave-length, therefore, 

 the former procedure is to be preferred. When, however, 

 the dispersion-curve is viewed as a whole, the limits between 

 which it may vary through experimental errors are not very 

 different in the two cases, and for the same reason no advan- 

 tage accrues from a steepening of the luminosity gradient 

 with an auxiliary plate of quartz or other optically active 

 substance. In the present investigation, where the electro- 

 magnet has been employed, the first method has been used 

 to obtain the final results, the second being applied in most 

 cases only to the preliminary or " pilot " plates. The follow- 

 ing lines were selected as being (a) sufficiently monochro- 

 matic, (b) of considerable intensity, and (c) nearly equally 

 spaced along the spectrum : — 



6708 (Li), 5893 (Na), 4958 (Fe), 4529 (Fe), 

 4046 (Fe), 3631 (Fe), 3306 (Fe), 3100 (Fe). 



Wratten Double-Instantaneous plates were used throughout, 

 the delicacy and fine grain of the film being well suited to 

 this class of work. The exposures necessary varied from 

 15 seconds to 2 minutes. 



The correction for the end-plates, being only just outside 

 the limits of experimental error, was obtained with sufficient 

 accuracy by calculation from the law of the inverse square 

 (8<x\~ 2 ), the mean rotation of glass for the D line being 

 taken as *016'. 



S2 



