260 Prof. R. ^Y. Wood on Screens 



a curve, quite similar to the curves laboriously constructed 

 from the readings obtained with the spectrophotometer *. 



The curve obtained with a solution of the nitroso in glycerine 

 is shown in Plate III. figs. 4 and 5. 



It will be noticed that after a certain thickness has been 

 passed we begin to get a noticeable absorption in the ultra- 

 violet, the form of the curve in this region being well shown 

 in figs. 3 and 4. The band in the blue and violet is, however, 

 so much heavier that, by employing a film of suitable thick- 

 ness, we can get complete opacity in this region, combined 

 with almost perfect transparency in the ultra-violet. The 

 nitroso is soluble in water, glycerine, ether, alcohol, bisulphide 

 of carbon, and many other fluids, and the region of heaviest 

 absorption varies somewhat with the nature of the solvent, 

 the shift of the band not. however, following Kundt's rule in 

 every case. 



A stained gelatine film on a quartz plate forms a fairly 

 suitable screen, if we do not wish to photograph below the 

 group of cadmium lines at wave-length 2314. It is opaque, 

 however, to waves much shorter than this. The glycerine 

 solution transmits down to the last cadmium line, X=2147, 

 and some other solvents appear to work equally well. 



In photographing the spectrum of the cadmium-spark in 

 the ultra-violet of the second order, with the fourteen-foot 

 concave grating, I found that the prolonged exposure of the 

 solution in glycerine to the light of the spark resulted in its 

 decomposition. Gas- bubbles formed in the thin quartz cell, 

 and by bridging across the space between the two plates 

 allowed the passage of blue and violet light. 



The same thing occurred with pure glycerine under a 

 quartz plate, while glycerine under glass was unaffected, 

 showing that the decomposition was caused by the extreme 

 ultra-violet. 



In addition to the formation of bubbles, a gradual bleaching 

 of the solution occurred. To obviate this difficulty I constructed 



* It is my intention to prepare a monograph on the absorption of a 

 large number of the aniline dyes, and other organic compounds such as the 

 nitroso-dimethyl-aniline, which have not been previously investigated. 

 The spectra will be approximately normal, all on the same scale, and will 

 extend from the C-line down to the end of the spectrum. They will be 

 photographed in the manner which I have described, and will, I hope, 

 make it possible for the spectroscopist or physicist to pick out at once the 

 combination necessary to produce any desired result. Preliminary experi- 

 ments are now in progress to determine the best form to give the apparatus, 

 and the most suitable source of light, and I shall be very glad of any 

 suggestions pertaining either to the apparatus or to particular substances 

 worthy of investigation. [A similar method of studying absorption 

 within the rano-e of the visible spectrum was emploved manv vears ago 

 by the late Dr.' Gladstone.— Ed. Phil Mag.'] 



