454 ON THE ABSORPTION OF LIGHT BY COLOURED MEDIA, 



riments on the violet end of the spectrum. I have met with 

 no coloured glass possessing a similar character. 



A solution of oxalate of nickel (free from cobalt) in ammo- 

 nia, may at first sight be mistaken for the ammonio-carbonate 

 of copper just mentioned. When set side by side with it, 

 however, it is characterised by the greater purity of its blue, 

 which amounts to an indigo tint, while the cupreous solution, 

 owing to the excess of the violet rays, has a strong blush of 

 violet. Examined with a prism, the solution of nickel is 

 found to absorb the most refrangible red, the yellow, green, 

 and extreme violet rays, with peculiar energy j but, what is re- 

 markable, the extreme red rays are transmitted by it with fa- 

 cility, and with their usual definite character. Their illumi- 

 nating power is too feeble, however, to affect the tint of the 

 liquid, and in very great thicknesses they are absorbed, leaving 

 only an image on the confines of the blue and violet. The 

 tvpe of this medium is as in Fig. 9. 



12. The yellow rays are absorbed with peculiar energy by 

 most purple media, but by none more decidedly than by a so- 

 lution of archil. This liquid, in small thicknesses, is of a neu- 

 tral purple hue, but becomes more ruddy by an increase of 

 thickness, and soon passes to the deepest red. (See Fig. 10.) 

 On the other hand, the various shades of purple, which pass 

 on one side into rose-colour, peach-blossom, and crimson, and 

 on the other into plum-colour and violet, arise from a mini- 

 mum of the spectrum in the space occupied by the green rays. 

 The various shades of purple glasses, the acid and alkaline so- 

 lutions of cobalt, &c. afford examples of this minimum. 



13. Hitherto I have supposed the illumination employed 

 TO be that of solar light, or ordinary day-light, and the above 



results 



