1857.] 



exhibited by Transmitted Light, 



339 



Salt of Chromium. 



posed across the spectrum, either of the sun, or of the electric light, 

 a remarkable configuration is observed, which shows that the 

 luminous bands of orange-red, of pale green, of orange, of yellow, 

 of blue, and of violet, are absorbed at different distances in the 

 order above given, while the extreme red penetrates any thickness 

 with almost undiminished brightness. Acetate of chromium, placed 

 in a hollow glass wedge, was seen to transmit the red, orange, green, 

 blue, and violet rays through a thin stratum ; yellow was absorbed 

 at once, violet very quickly, while the 

 maxima of luminosity were in the extreme 

 red, and about the junction of green and 

 blue, which in the Folar spectrum is mark- 

 ed by the dark line F. These blue and 

 green rays, however, which are transmitted 

 in such quantity at first that the solution 

 appears green to the unaided eye, are 

 gradually absorbed, while the red ray con- 

 tinues to penetrate the dense solution, 

 which of course assumes a red colour. A 

 solution of litmus was seen to transmit the 

 red^ green, and blue rays freely, the maxi- 

 nmm of absorption taking place between 

 the fixed lines C and D of the solar 

 spectrum : the addition of an alkali made little alteration beyond 

 facilitating the transmission of the blue ray ; while an acid dimi- 

 nished though it did not entirely retard it, causing the admission ^t 

 the same time of the orange ray, and shifting the maximum of 

 absorption to between b and F. As the red ray passes apparently 

 unchanged through a great thickness of any of these solutions, 

 neutral, alkaline, and acid litmus, all appear red if seen in sufficient 

 quantity ; indeed, paradoxical as it may sound, alkaline litmus is 

 then of a purer red than acid litmus, since 

 the latter transmits some orange light as 

 well. The various appearances of the pris- 

 matic spectrum, as seen through these 

 liquids in wedge-shaped vessels may be 

 easily copied by a draughtsman ; and, in 

 fact, coloured diagrams were displayed, 

 representing the prismatic images given by 

 most of the above-mentioned substances. 

 Some of these presented very characteristic 

 forms : thus, cochineal showed two maxima 

 of transmissibility, about B and G, pene- 

 trating far into the liquid, and two others 

 in the green space, which, however, were 

 speedily absorbed. Tincture of lavender, 

 too, gave a spectrum marked by absorption bands, which seemed 

 to coincide with the lines b, F, and G of the solar spectrum, though 



Cochineal. 



