66 PROCEEDINGS OF THE AMERICAN ACADEMY. 



manner, and found to agree very well with the results obtained with 

 prisms. To apply the method to tlie ultra-violet the observations were 

 made photographically with a small quartz spectrograph made by Fuess. 

 This instrument was furnished with a Rochou prism mounted immediately 

 behind the quartz collimation lens. This prism as furnished by the 

 maker of the instrument refused to transmit the ultra-violet, and I found 

 that the two halves had been cemented together with balsam, which I 

 replaced with glycerine. Some preliminary experiments were made 

 with selenium mirrors, as they were easier to handle than the fluid cell. 

 The light from a cadmium spark was reflected into the slit of the instru- 

 ment at various angles of incidence, and dark bands were found running 

 across the two polarized spectra furnished by the Rochon prism. I spent 

 some time endeavoring to extract data from these bands, and finally came 

 to the conclusion that either they were not due to the selective polariza- 

 tion by reflection, or else that the dispersion curve as determined for 

 selenium by means of the interferometer was in error. On experiment- 

 ing further I found that these bands were due solely to the rotatory 

 dispersion of the light in the quartz collimating lens, some colors being 

 rotated through such an angle as to be quenched in one spectrum by the 

 Rochon, and others in such a degree as to be absent in the other. I 

 mention this defect in the instrument as it may be of interest to others 

 working along similar lines. The proper design of the instrument should 

 have called for a collimating lens made of two thin lenses, one of right- 

 handed, the other of left-handed quartz. I remedied the defect in my 

 instrument by placing a flat plate of L. H. quartz immediatel}' behind 

 the lens. This plate had a thickness equal to the thickness of the lens 

 at its centre, and abolished the bands entirely, when the lens was dia- 

 phragmed down to a small area at the centre. 



I found, however, in working with the horizontal cell of fluid nitroso 

 that better results were obtained by using a Nicol prism in front of the 

 slit of the spectrograph, than with the Rochon prism. The Nicol was 

 made transparent to ultra-violet light by separating the two halves, 

 cleaning otF the balsam, and subtituting glycerine. 



Evidence of the very low value of the refractive index on the ultra- 

 violet side of the absorption band was obtained before any polarization 

 experiments were tried. The light of a cadmium spark was reflected 

 from the pool of licjuid nitroso at nearly normal incidence, and then 

 thrown into the spectrograi)h by means of a quartz total reflecting prism. 

 A series of spectra was taken, with times of exposure varying from 2 

 seconds to 3 minutes. By comparison of the different spectra it was 



