MISCELLANEOUS ABSORBING MEDIA. 



53 



169. Erbium Chloride Continued. 



depths, viz: 0.83, 1.13, 1.43, 1.73, 

 2.03, 2.33, 2.63, and 2.93 cm. In 

 other words, the thickness of the 

 absorbing layer was increased by 3 

 mm. between the successive photo- 

 graphic exposures. As has been 

 often remarked by other observers, 

 the solution in question has a very 

 large number of remarkably narrow 

 absorption bands. 

 For the depth of 0.83 cm. all of the 

 ultra-violet is absorbed from 0.20/* 

 to the cadmium line at 2880.9, while 

 for the depth of 2.93 cm. transmis- 

 sion begins near 0.300/11. The wave- 

 lengths of the maxima of the ab- 

 sorption bands, and the essential 

 characteristics of the bands, as ob- 

 tained directly from the original 

 negative, are as follows: 0.325/1, 

 0.35O/11, strong with a broad penum- 

 bra on both sides; 0.3555/x, faint; 

 0.3645/1, strong; 0.3662/t, faint com- 

 panion of the last; 0.3766/1, nar- 

 row and faint; 0.3792/1, strong and 

 sharp; 0.3875/4, faint, diffuse band 

 shading off gradually towards the 

 red ; 0.405/1, weak and sharp ; 

 0.4075/1, weak ; 0.416/1, faint, dif- 

 fuse band shading off towards the 

 red ; 0.419/1, faint ; 0.422/t, faint and 

 narrow ; 0.427/1, extremely faint 

 and diffuse band; 0.4425/1, faint; 

 0.450/1, comparatively strong and 

 narrow with a very faint com- 

 panion at the more refrangible side 

 and with a broad, hazy band near 

 the opposite edge ; 0.4675/1, very 

 faint; 0.4725/t, very faint and dif- 

 fuse ; 0.480/4, extremely faint ; 

 0.485/1, weak ; 0.4875/t, compara- 

 tively strong and narrow ; 0.491/1, 

 wide, hazy band shading off to- 

 wards the red; 0.5186/1, weak and 

 narrow ; 0.5205/1, narrow ; 0.5235/1, 

 strong and narrow ; 0.5365/1, weak 

 and broad ; and 0.5413/1, weak with 

 a broad, diffuse companion on the 

 side nearest to the red. 



170. Ethyl Alcohol. 

 See No. 148. 



171. Glycerine. 



A plane-parallel layer of glycerine 

 13.5 mm. deep absorbed all light 

 of wave-length less than 0.25/1 and 

 it produced a general weakening 



171. Glycerine Continued. 



of the continuous background as 

 far as about 0.33/t. The exposure 

 lasted for 1.5 minutes. 



172. Litmus. 



Figs. 83 and 84, pi. 21. 



In solution blue and red for the 

 neutral (or alkaline) and acid con- 

 ditions, respectively. 



Saturated. 



Angle, about 6 for both cases. 

 Depth o to 3.2 mm., approximately, 

 for fig. 83. 



The absorption of the blue solution 

 is suggested by fig. 83. Absorp- 

 tion was practically complete from 

 0.20/1 to about 0.28/1. From this 

 wave-length the absorption band 

 followed a gentle slope to about 

 0.42/1 for the greatest depth of so- 

 lution. A region of partial trans- 

 parency extended from 0.42/1 to 

 near 0.496/1. A band of absorption 

 began at 0.496/1 and had its max- 

 imum approximately at 0.53 i/i. 

 The spectrogram indicates the ex- 

 istence of intense absorption in the 

 orange and red. 



Fig. 84 gives the photographic record 

 obtained with an acid solution of 

 litmus. This solution absorbed 

 the greater part of the ultra-violet 

 region just as the neutral solution 

 did. On the other hand, the acid 

 solution exerted general absorp- 

 tion in the violet and blue, where- 

 as the neutral solution, of the same 

 depth, transmitted the light of 

 these colors. The maximum of 

 the band in the green was at 0.515/1 

 for the red solution. The displace- 

 ment of this maximum from 0.53 i/i 

 to 0.515/1 was probably exaggerated 

 by the variations of sensibility of 

 the photographic films for radia- 

 tions of different wave-lengths. 

 Fig. 84 recorded only weak ab- 

 sorption in the yellow-orange. Red 

 was transmitted. 



173. Methyl Alcohol. 

 See No. 148. 



174. Neodymium Ammonium Nitrate. 

 Figs. 96, 97, and 98, pi. 25. 

 Pink crystals. In solution pink. 

 Concentrated (filtered). 



For fig. 96 the solution was poured 

 into a quartz cell the ends of 



