Transparent only to Ultra-Violet Light, 261 



a small cell of quartz, by cementing two plates together, 

 with a space of about 0*5 millim. between them, the cell thus 

 formed being cemented to the bottom of a small thistle tube 

 with a very small bore. By filling the thistle tube with the 

 glycerine solution a flow took place through the cell at the 

 rate of about a drop every two minutes. This device worked 

 admirably, and gave no trouble at all, the cell being placed 

 close to the slit of the grating camera, in the path of the 

 convergent beam from the quartz lens. Another very satis- 

 factory screen can be made by dissolving celluloid (previously 

 boiled for some time in water) in amyl acetate, adding a 

 little nitroso, and flowing the solution on a quartz plate. It 

 is, however, opaque to the last two cadmium lines. 



The use of the screen necessitates considerable increase in 

 the time of the exposure, the amount varying from two to ten 

 or even twenty times, according to the density of the screen. 

 The strength of the glycerine solution must be adjusted 

 according to the work required of it : a strong solution 

 gives a wider band in the blue and violet, but diminishes the 

 intensity of the ultra-violet as well. In general the best 

 results are obtained when the blue line of wave-length 479 9 

 in the spark-spectrum of cadmium can be just barely dis- 

 cerned . 



hi Plate HI. fig- 1 the wave-lengths of the principal lines 

 in the spark-spectrum of cadmium are given for reference. 

 The action of the nitroso screen is wed shown in fig. 2, 

 the spectra being photographed with a small quartz spectro- 

 graph made by Fuess. The first seven spectra were taken 

 through the glycerine-nitroso cell which I have just described, 

 with the following times of exposure : 5, 10, 15, 20, 30, 15, 

 and (>0 seconds. The cell was then removed, and the follow- 

 ing six spectra taken with exposures 2f, 5, 10, 15, 20, and 

 30 seconds. A study of these spectra enables us to calculate 

 just what can be done with this screen, and the necessary 

 increase in the time of exposure resulting from its use. 



In fig 6, which is a negative, we have the absorption 

 spectra of the various components of the screen which 1 

 mentioned in the first part of this paper, taken with exposures 

 of 20 seconds each. 



The spectra were taken through screens as follows : — 



1. Nitroso in gelatine on glass (thin film). 



2. „ „ ., (thick film). 



3. ., „ on cobalt glass (thin film). 



1. ,, j, ,, ,, (thick film) strong 



ultra-violet absorption. 

 5. Dense cobalt <dass. 

 (i. TurnbulPs blue in gelatine. 



