262 Prof. E. AV. Wood on Screens 



7. Chance's '•Signal-green'" glass (two thicknesses). 



8. No screen. 3 seconds' exposure. 



9. Cyanine in canada-balsam. 



11. Anrantia in collodion. 



12. Signal-green glass (one thickness). 



These photographs were taken on an orthochromatic plate, 

 the yellow and yellow-green being compressed into the small 

 strip which appears alone by itself in No. 11. 



The utility of the nitroso screen in photography with the 

 concave grating is very clearly brought out in the photo- 

 graphs of the iron spectrum shown in Plate IV. These were 

 made with a 14-foot grating, with a glycerine nitroso cell 

 before the slit during one of the exposures. Figures 1 and 2 

 are from the same plate. Strip B in each was made through 

 the screen, and shows the ultra-violet of the third order, un- 

 contaminated by the blue of the second. In strips A, which 

 were made without the screen, the two orders are mixed. 

 Strips C were made through a glass screen, which cut off the 

 third order ultra-violet, leaving the blue of the second. I 

 have marked a few of the wave-lengths to aid in the identi- 

 fication of the lines. 



The times of exposure were for strips A and C ten minutes, 

 for B fifty minutes. 



The group of cadmium lines in the neighbourhood of 

 wave-length 2311 is, in the second order spectrum, mixed up 

 with a lot of blue air lines of the first order spectrum. The 

 separation of the two by the nitroso screen is well shown in 

 fig. 3, in which the two orders are shown superposed in strips 

 A, and the ultra-violet of the second order in strip B. The 

 exposures in this case were 15 minutes and 2 hours re- 

 spectively. 



Another screen which I believe may prove useful in astro- 

 physical work is made by combining nitroso-dim ethyl- 

 aniline with a small amount of the dye uranine, the latter 

 removing the bluish-green portion of the spectrum which 

 affects the photographic plate. By a proper adjustment of 

 the two in gelatine on glass, a screen can be formed which, 

 when used with an ordinary (i. e., not orthochromatic) plate 

 gives us a photograph made exclusively by ultra-violet 

 light, comprised between wave-lengths 345 and 365, a rather 

 narrow range. 



I have made a few photographs with a screen of this 

 description, which have brought out some interesting points. 

 In a photograph of the full moon, taken by ultra-violet light, 

 the contrast between the bright and dark areas is very strongly 

 accentuated, while in photographs of landscapes made in the 

 same way there is almost no contrast at all except between 



