﻿2 MORRIS- A IRE Y, Determination of Wave-lengths. 



avoid this absorption Stokes adopted as his source of 

 light an electric spark between aluminium electrodes, and 

 was then able to observe spectral lines as low as 1,852 A. 

 Miller, who was investigating ultra-violet spectra at about 

 the same time, was unable to photograph light with a 



o 



wave-length of less than 2,024 A. 



The result of this failure of photographic methods was 

 that subsequent investigators, Soret, Sarasin, and others, 

 relied solely on the fluorescent screen method, until Cornu 

 discovered that photographic plates could be prepared 

 which would register the last spectral line observed by- 

 Stokes. An important factor in his success was that he 

 recognised the strongly absorbing influence of air, and 

 accordingly designed his spectrograph so that the air path 

 traversed by the light in passing from the source to the 

 photographic plate was comparatively short. 



The aluminium double line at 1852 A remained the 

 boundary of the violet end of the spectrum until about 

 1889, when Victor Schumann, of Leipzig, began his 

 classical work. He soon rejected the conventional quartz 

 spectrograph in favour of one constructed with fluorspar 

 prism and lenses. It had already been found that the 

 white transparent variety of this mineral was at least 

 as transparent as quartz for ultra-violet light, and in 

 Schumann's hands it soon proved to be very superior to 

 quartz in this respect. An elaborate spectrograph * was 

 designed and constructed by Schumann, which could be 

 exhausted by means of a mercury pump, and all the 

 necessary adjustments of a spectrometer could be carried 

 out without admitting air to the apparatus. Schumann 

 discovered that ordinary photographic plates were quite 

 useless for ultra-violet work, as the gelatine in which the 

 particles of silver bromide and iodide are embedded 

 * V. Schumann, Wiener Berichte t 1S93. 



